H-P range (1940's)

The H-P engines

In this article I’ll attempt to summarize what little I’ve been able to discover about the H-P range of model engines from the early post-WW2 period in Britain. I hasten to point out that these engines have nothing whatsoever to do with the far later HP engines made by Hirtenberger Patronen of Austria from the early 1960's to the late 1980's! The coincidence of the two companies' trade-names causes some confusion, but it is merely a coincidence! Moreover, the pioneering British range came first by a good margin of time.............

The early post-war British H-P engines were marketed in relatively modest numbers by a firm known as Morbone Requisites which was located in Barnet, Hertfordshire, England. a little to the north-west of London. However, the range actually had its origins in early 1946 when the initial designs were manufactured and marketed by a different firm under the Atlas name. 

Unfortunately, both the Atlas and H-P model engines were very sparsely documented during their period of production. As a result, documentary source information is thin on the ground. Moreover, due to their comparative present-day rarity, opportunities for examining examples at first hand are rather limited. I’ll have to do the best that I can on the basis of the somewhat scanty information that is available, including a number of images extracted from various publications or provided by my valued colleagues.  

Indeed, I’m painfully aware that at present I actually know relatively little about the background to this rather obscure range. However, I would very much like to know more! Accordingly, this is one of those articles which I’m putting up here primarily in the hope that it may stimulate others into sharing what they know or are able to deduce from an examination of examples in their possession. Any additional information received will be added to the following text with both grateful thanks and full acknowledgement! This article therefore represents no more than a start in recording the history of this range - much of it remains to be written in the future. 

Thankfully, I do have a good representative example of the manufacturer’s work in the form of a Mk. III variant of the H-P 3.5 cc diesel. This provides an opportunity to evaluate the manufacturer’s capabilities at first hand as well as undertake the first-ever published test of the engine. 

As always when writing about any model engine range, it’s important to place the engines in their proper context. I’ll begin by attempting to do just that. 


Although large-scale commercial model engine manufacture had become very well established in the USA prior to the late 1939 onset of WW2 in Europe, the development of the industry in Britain was very much less rapid. In large part, this was due to the far smaller financially-empowered consumer base in Britain. The relatively high cost of model engines by comparison with the average Briton’s pre-war take-home pay packet initially restricted participation in power modelling to the better-heeled members of British society, some of whom were able to obtain engines from the USA.

Over time, the rapid pace of development did result in the increasing availability of good second-hand engines as their original owners moved on to the latest wunderwerk. However, it was not until the late 1930's that market conditions improved to the point where the commercial-scale series production of model engines in Britain began to be more widely perceived as an economically viable proposition.

In the interim, a number of individuals having both the required model engineering skills and equipment managed to produce their own engines on a very limited basis for their own use and that of their friends and club-mates. A number of these engines still survive today as “whatizit” units whose makers’ names are now lost to us. Many of these unattributed individually-built motors display outstanding workmanship which meets the best model engineering standards. Check out pp. 211-213 of Mike Clanford’s very useful if sometimes unreliable 1988 “Pictorial A-Z of Vintage and Classic Model Airplane Engines to see a representative sampling of these products of talented unknown makers, such as that illustrated here.

Of course, by no means every British resident who wished to participate in power modelling possessed the necessary combination of skill and equipment to make his own engine. Accordingly, a small but steadily growing domestic market for commercial ready-made engines did develop at a relatively early stage. Despite the limitations of this market, several British commercial model engine manufacturing ventures managed to get of the ground on a small scale during the early and mid 1930’s. These included E. Gray & Son of London, makers of the Grayspec and Grayson Gnome engines, as well as Cloud Model Aircraft of Dorking in Surrey (who manufactured kits as well as engines). However, both of these were relatively small-scale operations.

Mention of E. Gray & Son recalls to mind the strong circumstantial possibility that the Charlie Gray who reportedly designed the early E.D. engines (according to founding Managing Director Jack Ballard) and later designed the ill-fated JB range may well have been the "Son" of this pioneering company. This is presently an untested hypothesis based purely upon circumstantial evidence, but one that seems to me to be worth pursuing. I invite others in a better position than I am to follow up on this line of inquiry.  

Towards the end of the nineteen-thirties, a couple of British firms did manage to become established as somewhat larger-scale producers, most notably the Hallam company of Poole in Dorset, who produced a surprising diversity of model engines both before and after the war. There was also the remarkably prolific Leicester-based firm of Rogers & Geary, who produced the pre-war Spitfire 2.5 cc, Hornet 3.5 cc, Wasp 6 cc and (post-war) Stentor 6 cc engines for Model Aircraft Stores Ltd. of Bournemouth, later to become famous for their Veron range of model kits.

The onset of war in Europe on September 1st, 1939 effectively put a stop to this activity as the efforts of all of the nation’s more capable precision engineering firms became necessarily focused upon the war effort. The fact that the flying of powered model aircraft was officially banned for most of the war-time period had effectively stopped the power aeromodelling movement in its tracks.   

Naturally, all of the engines produced up to this point had been spark ignition types – indeed, British power modellers were widely referred to as "petrol enthusiasts" or (more colloquially) “petroleers”! However, the model compression ignition engine (commonly if improperly referred to as a “diesel”, which it isn’t!) had appeared with little fanfare in Switzerland during 1938 with the commercial advent of the ETHA diesels. This new type of model powerplant underwent considerable development during the wartime years in those European and Scandinavian countries which for one reason or another were not directly involved in the fighting. By the end of the war the model diesel had been developed into a formidable competitor for the “traditional” spark ignition models. Word of these developments quickly spread to Britain during the latter stages of the war.

The end of hostilities in mid 1945 brought about a complete change. The September 1944 lifting of the wartime ban on the operation of powered model aircraft had already paved the way for a significant upswing in the popularity of power modelling. An additional factor was the greatly enhanced level of “air mindedness” which had been fostered by the major role played by the Royal Air Force and its allied Air Forces in winning the war. Aeromodelling was on a roll!

Consequently, a considerable pent-up demand for model engines in Britain quickly became apparent. This was doubtless driven in large part by a completely understandable post-war desire to return to normalcy by resuming peace-time hobbies as quickly as possible. In addition, the novelty appeal of model engines during those early post-war years remained just as great as it had been during the pre-war period.

Although the cost of such engines remained highly significant in the context of the cash-starved British post-war economy, their novelty value encouraged people to save up and buy them anyway. Engines were in relatively short supply at the time, so any motor that would start, run reasonably well most of the time and keep on doing so for a while was pretty much assured of a buyer. 

In response to this demand, a number of new large-scale model engine manufacturing ventures quickly got off the ground, notably International Model Aircraft (IMA) with their FROG “bicycle spoke” model engine range, Mills Brothers with their original Mills 1.3 cc model and E.D. with their 2 cc sideport designs. I’ve covered the early post-war products of all three of those companies in separate articles. The long-established Hallam and Rogers & Geary firms both resumed model engine production in addition.

However, the field was by no means left entirely to the large-volume manufacturers. The end of the war had left a significant number of technically-skilled military and civilian personnel looking for fresh outlets for their talents. Those among them who were interested in model engines couldn’t all go to work for the relatively few major producers. Accordingly, a handful of them went into the model engine manufacturing business on their own account.

The result was the emergence in early post-war Britain of a surprising number of small-scale “cottage industry” model engine manufacturers, some of them working quite literally in the proverbial garden shed (or cow-shed in the case of the makers of the Clan engines from Fife in Scotland!). The names of Healy, M.E.C., Ace, E.P.C., B.M.P., Wilsco, Dyne, Clan, Comet, G.H.G., Rawlings, Gerald Smith, Leesil, Owat, Masco (castings only), E.R.E., Milford and Seymour, Hylda & Co. (Kalper) spring immediately to mind, and there were others. Indeed, there were almost certainly a number of such small-scale “artisan” ventures whose names are now lost to us.

However, there was an area in between the major producers and the small-scale artisan operations which lay open to modest-scale series producers of engines for marketing primarily in their own specific geographic areas. Firms which entered the market on such a mid-scale regional basis during the early post-war period included E. Reeves of Shifnal in Shropshire, Kemp Engines of Gravesend in Kent, Majesco Motors of Parkstone in Dorset, MS of Newcastle-upon-Tyne, Mechanair of Birmingham, the Model Aerodrome of Birmingham with their infamous Drome Demon, and Ten-Sixty-Six Products of Worcester.

Another such mid-scale firm was a key player in the story of the H-P engines with which we are primarily concerned here. This was the Atlas Motors company of Studham, near Dunstable in Bedfordshire. Interestingly enough, this was one of a number of small precision engineering firms owned by none other than D. A. Russell of “Aeromodeller” magazine fame. It has been credibly suggested that Russell’s good friend and close collaborator Lawrence H. Sparey may have had some involvement in the development of the Atlas models which first appeared in early 1946.  More of this below in its place.

Although by 1946 the model diesel was already well-known in British modelling circles, having reached a stage of development in Europe at which it was well able to compete with its spark ignition counterparts, many early post-war British manufacturers stayed with the tried-and-tested spark ignition format with which those involved were familiar, at least initially. In many ways this was an inevitable consequence of model diesel development in Britain having been sidelined during the war years due to other priorities, while development in a number of occupied or neutral Continental countries had proceeded rapidly. This of course left British diesel designers playing catch-up at the outset, a game which they were to play very successfully, as history shows. 

This being the case, it's scarcely surprising that many early post-war British manufacturers and modellers returned to the spark ignition engines with which they had become familiar prior to the onset of hostilities. A perfectly rational approach - start out with what you know and develop more technologically-advanced models as you go. E.D. and Mills were the most prominent exceptions to this, along with a very few others. 

Atlas was among the firms which confined themselves to the manufacture of spark ignition engines designed very much along pre-war lines. Given the relative scarcity of information about the activities of this company, there’s not a lot of detail that can be shared. As far as can be ascertained at this point in time, the Atlas range seems to have made its debut in early 1946 with the appearance of a few examples of a pre-production spark ignition model reportedly of 4 cc displacement. This prototype featured a blind-bored steel cylinder having integrally-machined cooling fins. 

Following a period of testing and assessment, this prototype was further developed into a 3.5 cc spark ignition model which made its appearance in series production form in mid 1946. This initial production variant of the Atlas sparker featured a cast alloy cylinder jacket in place of the integrally-machined cooling fins and blind bore of the prototype. This cylinder jacket incorporated the cylinder head in unit as well as the bypass passage, which was placed on the right-hand side of the upper cylinder unit (looking forward in the direction of flight). The complete upper cylinder unit was located on an installation flange formed at the top of the lower crankcase casting, being secured using four machine screws. 

The actual cylinder liner itself was presumably no longer blind-bored, evidently being inserted from below and sealing at the top with a suitable gasket. Three small holes were drilled through the jacket on the left-hand side opposite the bypass to provide egress for exhaust gasses. 

The Atlas engines seem to have quickly acquired a positive reputation among those who were able to obtain and use them. Surviving examples show the engines to have been quite well-made. Sales were evidently sufficiently encouraging that development continued, leading to the late 1946 appearance of the Atlas 3.5 cc Series II model. This model dispensed with the rather tricky cylinder assembly of its predecessor by employing a separate cylinder head which was attached using machine screws. In other respects, this model was seemingly little changed from its predecessor. This was to prove to be the final model to be released under the Atlas name. 

Although they never manufactured a diesel model under their own brand name, Atlas Motors did became involved with model diesels in mid 1946, when they began to advertise sets of plans, castings and materials for the well-known 5 cc diesel which had been developed by D. A. Russell’s good friend and resident “Aeromodeller” engine tester Lawrence H. Sparey. This engine had been featured in the magazine, hence being a popular subject for home construction, as it remains today. 

By October of 1946 the company had added a similar kit for the home construction of their original 3.5 cc Series I spark ignition model (which for some reason they advertised as being of 3 cc displacement – the only mention that I can find of such a displacement in the context of this series).  The price of this kit was £2 2s 0d  (£2.10). The complete engine ready to run was available at a price of £7 10s 0d  (£7.50). 

It was shortly after this point in time that a change of manufacturer led to the establishment of our main subject range, the H-P engines. In early 1947 both the manufacture and the the marketing of the Atlas range were taken over by others. To reflect the identity of the new manufacturing firm, the name of the range was changed at this point to H-P, consigning the Atlas marque to model engine history. The marketing of the re-named engines was taken on by the previously-mentioned Barnet firm of Morbone Requisites. 

The H-P Models 

At last we encounter some acknowledgement of these engines in the contemporary British modelling media. The H-P engines from this point onwards were included in the list of British motors which formed an appendix to Ron Warring’s early 1949 book Miniature Aero Motors. Based upon a careful review of its coverage, this book actually appears to have been compiled and written in late 1948. 

One might think that this would do much to clarify the identification of the various H-P models. However, this is not the case. Warring seems to have been singularly ill-informed regarding the H-P engines – in fact, the evidence strongly suggests that he never actually had a chance to examine any of them personally. This would explain for example why he was unable to provide any bore and stroke figures, which would have done much to end the confusion regarding displacements. It seems that he was largely presenting information extracted from the manufacturer’s own promotional material. 

Presumably as a result, none of the H-P engines were included in Warring’s separate table of detailed construction data for British engines. This seems to confirm the suspicion that Warring had not had an opportunity to obtain such data, either from the makers or by direct observation. In particular, he had evidently had no opportunity to take dimensional measurements. This presumably also explains the fact that although the H-P range was included in his table of British motors, the H-P engines were not mentioned at all in the main text. 

Indeed, as it turned out, their inclusion in the appendix to Warring’s book represents the one appearance of the H-P marque in contemporary modelling literature. The H-P diesels arrived on the scene too late to be mentioned by D. J. Laidlaw-Dickson in his ground-breaking late 1946 book Model Diesels”. Fair enough, but they were certainly around when Col. C. E. Bowden was writing his late 1947 book Diesel Model Engines. Despite this, the H-P diesels were not as much as mentioned in either the first or second editions of Col. Bowden’s book. One wonders why ………. 

This lack of contemporary documentation brings up a point that needs to be made right at the outset. It has to be admitted that considerable confusion has always existed regarding the identification of the various H-P models and even their displacements. To take just one example, the model identified by Mike Clanford as the H-P Series II diesel in his very useful but at times misleading 1988 Pictorial A-Z”  book is cited by O. F. W. Fisher in his earlier 1977 Collector’s Guide as the Series I model! On architectural grounds (see below), I have to side with Clanford on this point, but the doubt remains. 

There’s also the issue of displacements. Figures of 3.5 cc and 4 cc have been freely applied by different writers to the same model, making the secure classification of these engines extremely difficult. It seems possible that the same basic designs were offered in both displacements – indeed, there are a few very thin shreds of evidence to support this possibility, as we shall see below. However, one might question why an increase of only 0.5 cc would be viewed as sufficient justification for the release of a different variant. Only the opportunity to examine and measure a representative sample of these engines could throw any light upon this question. 

With the above cautionary comments in mind, let’s take a look at the various models which are known to have been marketed by Morbone Requisites under their new H-P brand name. Most people’s first question when discussing this range would probably be – why H-P?!? Isn't that the name of a very popular British "brown sauce"?!? Well, yes, but we're dealing here with model engines, not condiments! The change in the name of the range clearly implies a concurrent change of manufacturer, but the initials bear no relationship whatever to the marketing firm's company name. 

The confusion in this case arises primarily from the previous unquestioning identification (by Mike Clanford among others) of Morbone Requisites as the manufacturers of the H-P engines. In reality, it would seem that they were only the marketing agents for the range. A number of anecdotal sources have combined to identify the actual manufacturers of the H-P engines as a firm called Henleyston Products. In his excellent and highly recommended pictorial catalogue entitled "British Model Aero Engines 1946 - 2011", Ted Sladden identifies the manufacturers as Henleyston Products of Bruce Road in Barnet, Hertfordshire. Given the role of the Barnet-based Morbone Requisites in marketing the range, this makes perfect sense to me. This identification also makes immediate sense of those pesky initials – H-P stands for Henleyston Products!

The first model to be marketed under the H-P banner in early 1947 seems to have been in effect nothing more than a clone of the late 1946 Atlas 3.5 cc Series II spark ignition model as manufactured by the original Atlas company. It was however marketed from this point onwards as an H-P product. The H-P identification was stamped onto the backplate of these units. 

Here we encounter the first anomaly when it comes to the identification of these engines. Clanford claims that this model’s Atlas heritage was acknowledged through its being presented as the H-P 3.5 Series III model, thus continuing the Series designation begun by Atlas. However, this is at variance with the data presented by Warring, who made no mention of an H-P Series III sparker, instead listing both Mk. I and Mk. II (not Series I and II, note) variants of the H-P spark ignition model. 

These two accounts are clearly contradictory. Given his far greater proximity to the production period of the H-P engines, I’m inclined to side with Warring on this issue. The fact that my own example of an H-P engine bears a factory-stamped Mark number as opposed to a Series designation seems to confirm that the company switched from Series numbers to Mark numbers when they acquired the Atlas range, as implied by Warring. 

Warring’s version is further supported by the fact that the H-P spark ignition model is undoubtedly encountered in not one but two quite distinct variants. The first of these (presumably Warring’s H-P Mk. I model) is essentially identical to the Atlas Series II design. It retains the three drilled exhaust apertures placed on the left side of the cylinder opposite the single bypass and transfer porting, which was still located on the right hand side. It looks more or less identical to the Atlas Series II model. Eric Offen’s illustrated example, which is unquestionably an H-P Mk. I model,  bears the serial number M-411. The stamped initials H-P on the backplate confirm its H-P origin. 

The other variant (presumably the later H-P Mk. II model) features completely redesigned cylinder porting. The upper cylinder casting now incorporates a single bypass formed at the front rather than at the right-hand side as in the previous model. This allows the provision of exhaust ports on both sides of the cylinder rather than only on the left-hand side as in the former design. Moreover, these ports are made far larger in area, being created by milling rather than being drilled. It was this model which evidently formed the basis for the H-P diesels which soon followed (see below). 

Unfortunately, it was also this model which was identified by Mike Clanford as the H-P Series III design. I’m personally convinced that this is incorrect and that it is in reality Warring’s H-P Mk. II model. Its later date than the unit which I believe to be the Mk. I is effectively confirmed by the fact that all of the later diesel models used the same re-configured upper cylinder design with a forward bypass and twin exhausts, one on each side. 

One matter upon which Warring’s evidence unfortunately muddies the waters is that of displacements. Warring cited the displacements of the Mk. I and Mk. II variants of the H-P spark ignition model as 3.5 cc and 4.0 cc respectively. This appears to imply a belief by Warring that the two Mark designations represented different displacements as well as being different designs. In effect, he seems to be expressing a belief that the H-P spark ignition model was made in both 3.5 cc and 4.0 cc displacements, with the different Mark numbers distinguishing between the two. 

Of course, nothing to do with the identification of the H-P engines is ever straightforward or unambiguous!  Our friend Fisher adds to the confusion by specifically referring to the H-P Mk II sparker as a 3.5 cc model, in direct contradiction to Warring. AAARRRGGGHHH!@?!! This issue of displacements is one of the great uncertainties relating to the secure identification of these engines – we get no help at all from the various literature sources, or for that matter from the engines’ external features. 

In keeping with its name, Clanford cited the displacement of the H-P 3.5 Series. III sparker (which I believe to be the H-P Mk. II model) as 3.5 cc (.212 cuin), although he presented no bore and stroke dimensions to back this up. According to Warring, the engine (which he called the H-P Mk. II, remember) weighed 7.5 ounces (213 gm). Its selling price was given as £7 10s 0d – a serious chunk of change in early 1947 Britain. 

This highlights a point which needs to be made here. As of early 1947 model engine prices had not begun their subsequent readjustment to more closely reflect the purchasing power of the average potential customer in Britain, at least in the larger displacements. Companies such as IMA (FROG) and E.D. were leading the way in that direction with their far more economically-priced models, but at this point those companies were strictly involved with smaller engines of 2 cc or less. If you wanted a larger displacement engine, you had to look elsewhere and be prepared to pay for the privilege. 

British modellers had to wait until 1949 for the appearance of less expensive engines in the displacement categories above 2.5 cc (0.15 cuin.). When this did begin to occur, there was of course a price to be paid – few of the smaller-scale or “artisan” manufacturers listed earlier were able to remain competitive. It was the economy of large-scale production that allowed the achievement of lower prices by the major manufacturers. Those lower prices meant that few of the garden-shed and low-volume operators could make economic sense out of remaining in business. 

By 1950, most of the smaller-scale manufacturers listed earlier (including H-P) had vanished from the scene, significantly reducing the diversity of model engines remaining available in the marketplace. The British model engine marketplace was never again to offer the number and diversity of different marques which had peaked during the latter part of the 1940’s. 

1947 marked another watershed for the British model engine manufacturing industry, because it was in that year that the spotlight shifted decisively from spark ignition to diesel. From 1947 onwards, few manufacturers introduced new spark ignition models – in fact, many existing sparkies were withdrawn from production in that year or soon thereafter. There were exceptions, of course, most notably in the large racing engine field with spark ignition versions of such units as the Rowell 60, Ten-Sixty-Six Conqueror and Nordec R10, but these were the exceptions – from this point onwards, diesels dominated the British modelling scene, as they were to do for several decades to come. 

It’s very clear that the promoters of the H-P range recognised this emerging trend, because they appear to have been very quick to switch their development focus from spark ignition to diesel. The two spark ignition models noted above (by whatever name!) were the last to appear under the H-P banner - from mid 1947 onwards, the manufacturers focused their attention strictly upon diesels. 

The first H-P diesel was the 3.5 cc Mk. I model (or Series I if you believe Clanford on this point). This model seems to have reached the market in mid 1947. All sources agree that this engine was a direct development of the earlier H-P 3.5 cc Mk. II spark ignition design. 

Like its spark ignition companion in the range, this model was included in Ron Warring’s table of British motors in his previously-mentioned book. However, its identification was greatly confused (yet again!) by some inconsistent data which appears in Warring’s table. For one thing, the engine was identified as the H-P Mk. IX model. I believe this to be an uncaught misprint, since it is in no way consistent with what is known of the H-P nomenclature in general. 

To make matters even more confusing, Warring cited a displacement of 0.208 cuin., which converts to 3.4 cc. However, he then muddled things further by rendering this displacement in its metric form as 3.9 cc!! Clearly another example of an unedited misprint – the two figures cannot be reconciled. 

Unfortunately, Warring once again stopped short of providing bore and stroke data, leaving us with no way of resolving this anomaly. He did however quote the engine’s weight as 6.5 ounces. The selling price was recorded as £6 10s 0d – a worthwhile reduction from the price of the earlier sparker. 

This unit once again raises the issue of model identification mentioned earlier. In his 1977 “Collector’s Guide”, O. F. W. Fisher claims that the H-P Series I diesel (as he calls it) featured four studs which secured both the cylinder and the head.  By contrast,  Clanford’s illustration of the four-stud variant is identified as the H-P Series II diesel (see below), with a cited displacement of 4 cc as opposed to the 3.5 cc displacement of all previous models in the Atlas/H-P series. 

I have to say that the architectural evidence seems to support Clanford’s model identification. I base this impression upon the fact that the previously-illustrated engine which Clanford identified as the Series I model features a crankcase having a rather skimpy-looking main bearing housing which seems to be externally machined to accept a timer. In fact, this looks like the same component which was used on the earlier H-P Mk. II spark ignition model.

This appears to confirm that the first H-P diesel was simply a diesel conversion of the sparker, just as claimed by Clanford. Apart from the omission of the timer and the addition of a contra-piston, the most obvious external changes were the addition of a pair of strengthening buttresses at the sides of the upper cylinder casting below the exhaust ports as well as a vertical extension of that component to accomodate the extra cylinder length necessitated by the addition of a contra-piston.  

Although he does not give a specific displacement for this model, Clanford’s quite reasonable contention that this was simply a diesel version of the established 3.5 cc sparker implies that it too had a displacement of 3.5 cc. I see no reason to doubt this interpretation, which is generally consistent with the 0.208 cuin. figure given by Warring. 

Later in 1947, a revised diesel model was introduced as the H-P Mk. II diesel. The unit illustrated by Clanford in his "A-Z" book is captioned as the H-P Series II diesel, but we need not split hairs over the question of Mark versus Series numbers! In structural terms, this model seemingly represented an effort on the part of the manufacturer to beef up the design somewhat. The most obvious external evidence of this is the switch from a machine screw assembly for the cylinder and head to the use of four sturdy-looking steel studs which secure both components. 

And it didn’t stop there – the main bearing housing too was considerably strengthened through the provision of a conical expansion at the crankcase end, which served in effect as a 360 degree buttress to increase the strength of the bearing. It must be admitted that this was a logical move – the un-reinforced configuration seen on the earlier converted sparker does seem rather vulnerable to crash damage. 

It is this change in the configuration of the main bearing that convinces me that the four-stud model did indeed follow on the heels of the original spark ignition conversion. It’s hard to see the designer going from a far stronger main bearing to a significantly weaker one and then back again. 

While the modification to the main bearing housing is readily understood, the same cannot be said of the switch from machine screw assembly to studs for the cylinder. The revised design seems to represent a step backward by adding an additional element of complexity along with a negative impact upon the engine’s aesthetic appeal. However, there must surely have been a reason for this design change. 

Here we may be seeing a straw in the wind to support the notion that at least some of these engines were indeed made in 4 cc displacements. Clanford specifically claims that the displacement of the illustrated model was 4.0 cc, although he includes no measurements to support this statement. If an increase in displacement from 3.5 cc to 4 cc was in fact contemplated, there would have been good reason for the designer to anticipate increased stresses upon the working components and to consider possible ways in which such increases could be resisted. The use of the studs for cylinder retention as well as the stronger main bearing housing might well be seen as logical measures to meet this perceived requirement. 

As ever, the only way to resolve this question would be to take measurements from an actual example. If anyone reading this article has an H-P diesel having a precisely measured and confirmed displacement of 4 cc, I’d be immensely grateful to hear from you!! 

Early 1948 saw the introduction of what was to prove to be the final H-P model – the so-called 4 cc Mk. III diesel (or Series III if you go with Clanford). Both Fisher and Clanford are (for once!) in agreement with the engine’s Series III identification, although Warring stays true to the Mk. designation which he applied to the other H-P models, designating this unit as the H-P Mk. III diesel. Since my own example is clearly factory-stamped "H-P MK 3", I'll go with Warring on this point...........

All three sources also agree upon the engine’s displacement, which they all cite as 4.0 cc. Finally we have unanimity all round! However, once again none of the above sources provides any bore and stroke data to allow the stated displacement figure to be checked. 

The H-P Mk. III (or Series III) diesel was actually little changed from its predecessor in external design terms. It retained the strengthened main bearing housing of its predecessor. The most obvious visible change was its reversion to the far neater machine screw assembly for the cylinder and head. Presumably the beefed-up and more costly construction using steel studs had been found through operational experience to be unnecessary. 

But now we get back (as ever) to the thorny issue of displacement. In this case, the factor that messes things up is my own engine number D-654. If measurements taken from this example confirmed the 4 cc displacement claimed by all of the above-noted sources, we’d be home and dry. The problem is that they don’t! Based upon precise and repeated measurements, it turns out a) that the claimed displacement is a considerable exaggeration, at least in this instance; and b) that this particular engine is slightly over-square – very unusual for a sideport diesel of the period in question, in which long-stroke geometry was generally favored. 

The measured example has nominal bore and stroke dimensions of 21/32 (0.656 in. - 16.67 mm) and 5/8 (0.625 in. - 15.87 mm) for a design displacement of 3.46 cc. The actual carefully measured (and re-measured) bore and stroke dimensions of this example are 0.658 in. (16.713 mm) and 0.622 in. (15.798 mm) respectively – well within reasonable limits for small-scale production using the presumably worn-out wartime equipment which most early post-war British manufacturers were forced by economic considerations to use. 

Regardless, this particular example of the “4 cc” H-P Mk. III (or Series III) diesel is indisputably a 3.5 cc unit! As hypothesized earlier, it is actually possible that the Mk. III diesel was made in both 3.5 cc and 4 cc form, being stroked out to 4 cc and thus becoming a long-stroke engine. A modest increase in the nominal stroke to 23/32 in. (18.26 mm) while retaining the same bore would yield a displacement of 3.98 cc. although one would have to wonder why .......... However, there’s currently no hard evidence to settle this question one way or the other. 

Warring stopped short of quoting a weight for this engine, thus reinforcing the impression that he probably never examined one personally. The illustrated example weighs a ponderous 8.08 ounces (229 gm) - considerably heavier than the contemporary Reeves 3.4 diesel (6.5 ounces), for example. Warring quoted a selling price of a very hefty £8 1s 0d. Most interestingly, he also included the notation that as far as he was aware the engine was still on the market at his time of writing (likely late 1948). 

Production of this model seems to have continued into 1949, but almost certainly ceased quite early on in that year as far lighter, less costly and more powerful models progressively appeared from competing manufacturers. It seems highly doubtful that the numbers produced reached four figures. Whatever the final tally, the termination of production of this model signaled the end of the H-P range after only two years on the market under that brand-name. 

The H-P engines all appear to have borne serial numbers. Eric Offen’s H-P Mk. I sparker bears the number M-411, while my own H-P Mk. III diesel is numbered D-654. Fisher illustrated a seemingly identical Mk. III diesel which bore the number D-218. The letter evidently identifies the particular series to which the engine belongs, while the number presumbly indicates the engine's place in the production sequence for that series. In each case, the letter is stamped beneath the right-hand mounting lug (looking forward in the direction of flight) while the number is stamped in the corresponding location beneath the left-hand lug. 

Taken together, these numbers seem to imply that at least 411 examples of the sparkie and 654 examples of the diesel were made. If we factor in the Mk. II sparker and the Mk. I and Mk. II diesel models, we might reasonably postulate a total production figure of some 2,000 engines, taking all known H-P models into account. This in turn implies a relatively small-scale manufacturing operation with only a handful of employees producing some 80 engines monthly on average over the two-year market tenure of the H-P range. 

The fact that Fisher quoted a displacement of 4 cc for his illustrated diesel example is interesting. It would appear from the admittedly scanty evidence available so far that the manufacturers used a different letter to denote engines in a given series. This being the case, the fact that the two diesel examples (mine and Fisher's) are serial-numbered in the same sequence seems to me to make it unlikely that different displacements were involved – surely a larger 4 cc model would be assigned to a different serial numbering sequence having a different identification letter? One wonders if Fisher actually took measurements from his example ………… 

That’s about all that can be gleaned about these engines from a review of the scanty, incomplete and often contradictory documentary evidence. To learn more we have to look at the engines themselves. Let’s make a start by examining the single example of an H-P engine that is currently on hand for examination – the Mk. III diesel, in this case of confirmed 3.5 cc displacement. 

The H-P Mk. III Diesel Described 

My illustrated example of the engine is in near-mint condition, seemingly having had little use, although it has clearly been mounted and run. This particular engine was given to the previous owner years ago by one Terry Everit, who used to work part time at H-P in the late 1940’s. I am thus only the engine’s third owner in over 70 years! 

This example bears the serial number D-654, with the three-digit number being stamped under the left-hand mounting lug (looking forward in the direction of flight) and the letter “D” stamped on the underside of the right-hand lug. The backplate is stamped with the identification “H-P MK 3” as written. No identification markings are cast onto the engine at any point. 

The H-P Mk. III is in essence an entirely typical sideport diesel of its era. Its main distinguishing feature is its use (in this 3.5 cc example) of short-stroke power train geometry. As noted earlier, the measured bore and stroke of this example are 0.658 in. (16.713 mm) and 0.622 in. (15.798 mm) respectively for a displacement of 3.47 cc (0.212 cuin.). The engine weighs in at 8.08 ounces (229 gm) -  a lot of weight for a 3.5 cc engine, even by 1947 standards. 

The engine’s main external features are clearly visible in the attached images. As can be seen, the H-P is built up around a main casting consisting of two separate components – the crankcase itself with its integrally-cast main bearing, and an upper cylinder casting which incorporates the bypass and exhaust porting, the inlet stub and the cooling fins above the exhaust port level. The upper and lower castings are united by an installation flange below exhaust port level, being secured by four machine screws. 

The steel cylinder liner drops into the upper cylinder casting, semingly being shrunk into position. It is topped with a turned alloy head which is secured once again by machine screws. The contra piston is also of steel. Here we encounter one of the engine’s more unusual design features. The comp screw is not threaded though the cylinder head in the normal manner – instead it simply passes from below through an unthreaded drilled hole in the head and is located in position vertically by a shoulder formed above its lower threaded length. It is thus free to turn in either direction while remaining at exactly the same elevation relative to the head. The external cap of the comp screw is a separate component which is retained by the single control arm. 

So how does this comp screw engage with the contra piston?  Very simple – below its locating shoulder, the comp screw is externally threaded 2 BA, but with a left-hand thread! This thread engages with a similar 2 BA female left-hand thread formed in the centre of the contra piston. Hopefully a moment’s thought will convince the reader that the effect of turning the comp screw is not to change the elevation of the screw itself as in the usual case (since that is fixed), but to move the contra piston up or down due to the action of the comp screw’s threaded lower end on the internally-threaded hole in the centre of the contra-piston. 

A little further thought should convince the reader that the left-hand thread will result in the comp screw working exactly as in the more conventional arrangement – turning it clockwise (viewed from above) will increase compression, and vice versa. Excellent justification for the use of a left-hand thread! The other effect of this arrangement is that it provides positive control of the contra piston in either direction – turning the comp screw anti-clockwise (viewed from above) will pull the contra piston up on its thread with no help needed from internal gas pressure in the cylinder. In other words, this design gives the comp screw a positive two-way effect. This could be quite handy during starting. 

As received, the contra piston in this example was extremely loose. This may be explained by the fact that it is made of hardened steel – a contra-piston of this material has a well-known tendency to stick in a steel bore when hot. Steel contra pistons were often fitted a bit on the loose side to compensate for this. In this instance, the designer may also have been relying on the two-way positive control provided by the comp screw arrangement to keep things in place when running. 

If so, the manufacturers went a little too far with this example! A test run of the engine soon after it arrived showed that although the contra-piston sealed well enough, it would not hold a setting when the engine was running, even when hot. This made the establishment and maintenance of precise settings very difficult, even under bench test conditions. For use in the field, one would have to devise a means of securing the comp screw once a setting was established. 

For this reason, I ended up making a properly-fitted replica contra piston out of cast iron. This worked perfectly on test, allowing the engine to be adjusted very precisely, after which it would hold its setting securely. 

The final comment regarding the compression adjustment arrangements relates to the fasteners which secure the head in place. These have a thread of 8 BA, the female threads for which are easily stripped if over-tightened – please take care!! Five of the six fasteners are conventional machine screws, but the sixth is a slot-headed steel stud with an 8 BA spigot protruding from its base. The intent is clearly that after the required range of compression settings is established, the stud can be relocated to any of six available locations in order to limit the range of available compression settings. A good design, both to minimize the possibility of creating a hydraulic lock and to retain the correct starting and running compression range.  

Moving downwards, the exhaust chores are handled by two monumentally large milled apertures placed one on each side. The transfer ports consist of a row of three small drilled holes placed at the front of the cylinder between the exhaust ports. These are fed through a generously-dimensioned bypass passage formed in the interior of the crankcase at the front. The transfer ports overlap the exhaust ports almost completely. 

The induction port at the rear of the cylinder between the exhaust ports is a milled rectangular opening of considerable area. In conjunction with an intake tract of seemingly adequate diameter, the engine appears to be well provided as far as breathing is concerned. 

I was unable to fully dismantle the engine since the backplate appeared to be screwed home “for keeps”, resisting all my efforts to free it by fair means. However, I can report that the flat-topped cast iron piston is a relatively lightweight component having quite thin walls. I would actually have preferred to see a bit more metal left to form the bosses for the solid steel gudgeon (wrist) pin. The 0.125 in. (3.175 mm) pin looks to be very much on the skimpy side for an engine of this displacement, although it too contributes to a lower-than-average reciprocating weight. It appears to be a press-fit in the very short bosses. In my personal view, this is actually the weakest link in the design chain for the engine's working components. 

The piston drives what appears to be a one-piece steel crankshaft through a seemingly very sturdy cast alloy con-rod. The bearings at both ends of this component are of more than adequate length. In particular, the full-length small end bearing doubtless contributes to the ability of the rather skinny gudgeon pin to deal with the loads imposed upon it. The standard of fitting is exceptionally good – almost no play can be detected. 

The crankweb is equipped with sizeable cutaways on each side of the crankpin for counterbalance. Taken in conjunction with the short stroke and the lightweight reciprocating components, I would expect this engine to produce lower levels of vibration than might otherwise be expected. 

Another somewhat questionable design feature (in my personal view) is the means of keying the steel prop driver to the shaft. This is accomplished by means of a small set-screw which engages with a blind hole in the forward extension of the crankshaft. The idea is presumably that in a hard crash the set screw will shear, thus preventing the development of potentially damaging stresses in the shaft itself.  However, the screw seems to be a bit on the undersized side, hence appearing vulnerable to wear even in normal service. Moreover, the required hole in the shaft appears to create a stress concentration point in that component which may defeat its original purpose.  

The carburetor/tank assembly is connected to the cylinder intake stub through an externally threaded brass nipple. This nipple engages with female threads in both the intake stub and the delivery end of the carburetor intake tract. It is first threaded tightly into the intake tract and is then screwed into the intake stub as far as desired, being set in the desired position relative to the rest of the engine using a lock-nut. The tank can thus be positioned in any desired orientation. The actual tank is one of H-P’s “trademark” black plastic screw-in components which had been used from the outset going back to the Atlas era. 

E.D. used a similar carburetor mounting system on the early variants of their 2 cc Mk. II diesel. However, they experienced a lot of trouble with breakages, hence switching to a more sturdy arrangement quite early on. I’d expect the H-P component to be similarly vulnerable. However, the consequence of a breakage is merely the fitting of a new replacement brass nipple.   

The quality of the engine’s construction is very high, particularly where it counts. All working fits are exceptionally good. With the proviso that the seemingly undersized gudgeon pin might give trouble over time, I’d expect this engine to give long and dependable service if handled respectfully. If they were all as well-made as this example, the H-P range would certainly count as one of Britain's higher-quality marques from the early post-war period. 

Fair enough – so here we have a very well-made and generally well-designed model diesel, albeit one which could benefit by going on a diet! How does it run?? Let’s find out!!  

The H-P Mk. III Diesel on Test 

I’ve actually had this engine for quite a while, but had never got around to doing a full test due to the very loose contra-piston which was found to preclude the establishment and retention of stable compression settings. On its one running session in my hands, the engine had turned an 11x5 Taipan prop at 6,300 rpm. However, I was forced to maintain the compression setting by hand, making precise settings very difficult to establish and sustain while taking speed measurements. I felt at the time that the engine would probably do slightly better than this if provided with a better-fitting contra-piston. As related above, I made such a component prior to undertaking a full test of the engine.

I was expecting this engine to be a low-speed torque producer rather than a powerhouse, selecting my props accordingly. As it turned out, the range of props that I did select neatly bracketed the region of the engine's peak output. Since the fitting of a new contra piston meant that I didn't have any idea of the range of compression settings that would be required during the test, I began with a standard 8 BA machine screw in place of the compression stop pin which normally does double duty as one of the head retaining screws. 

I installed the H-P in the test stand, fitted an 11x7 Zinger wood prop and got right down to it. Since the engine had not been run for some years, it was rather full of storage oil. A small prime was required to get it to fire up and blow this out of the system. I also made life difficult for myself by guessing very much on the rich side as far as the needle setting went, flooding the crankcase as a result. However, it didn't take long to work through these difficulties and get the H-P running. Once the required range of compression settings was established, I re-installed the stop pin in the appropriate location. 

From that point onwards, it was all systems go!! The engine proved to be almost absurdly easy to start, never requiring another prime either hot or cold. A single choked flick was the only necessary preliminary, after which a few more flicks invariably resulted in a start. I found it best to open the needle about a quarter turn for cold starting, but hot re-starts were immediate at running settings. 

The test was further enhanced by the engine's excellent response to both controls. My replacement contra-piston proved to be perfectly fitted, holding its settings firmly while remaining readily adjustable at all times. The needle gave very positive feedback without being overly sensitive, making the optimum needle setting very easy to establish. The split thimble on the needle held its setting securely at all times.

Once set, running qualities were all that could be desired. The engine ran very steadily without missing a beat. It also displayed no tendency to sag as it warmed up. The exhaust residue was completely clear at all times, indicating good combustion characteristics along with very well-mated moving parts. No trace of any issues with the engine's working components manifested itself at any time during a long and at times arduous test. In particular, that very skinny gudgeon pin behaved itself perfectly! 

A particularly welcome finding was the fact that (as anticipated) vibration levels were relatively low, no doubt due to the combination of a short stroke, comparatively lightweight reciprocating components and a generously counterbalanced crankweb. I had no compunction about running the engine well past its peak - indeed, it was quite happy to run at such elevated speeds, albeit of course with a reduced power output. Starting remained exceptionally good on all props tested.  

Overall, I'd rate this as one of the most enjoyable tests that I've conducted for some time. I was enjoying running this fine engine so much that I forgot for a while that I was supposed to be checking it out on different props! I did however get back to the task at hand, testing a range of loads which proved to bracket the peak very nicely. The figures obtained were as follows, together with the derived power curve:




12x6 APC

11x7 Zinger wood 

11x6 APC

10x7 TF  wood

10½ x6 APC

11x5 Taipan

10x7 APC

10x6 APC

10x4 APC



















As can be seen, the H-P delivered a very solid performance by the standards of its day, developing some 0.130 BHP @ 6,400 rpm. Plenty of low-speed torque on tap! The range of props which it would turn effectively was quite impressive - it was developing over 0.120 BHP at all speeds between 5,200 and 7,200 rpm. Any prop that held it to within that speed range in flight would be quite effective in service. For free flight use, I'd probably opt for something like a 12x5, while in control line service a 10x8 would probably work well. 

The engine came through its test with flying colours, seemingly ready to keep running as long as required and then some. Although it is undeniably a bit of a lump for the power that it develops, I'd have to rate this as a pretty solid effort on the part of Henleyston Products!


If my own tested example is any indication, the H-P range of model engines were products of a manufacturer who worked to very high standards and had a good grasp of model engine design as it existed in 1946-47. The downfall of the range almost certainly came about in large part because the company lacked either the resources or perhaps the incentive to develop the new updated models which would have been required for the range to remain competitive.

Another factor which doubtless played its part was price. It would appear that the prices of the H-P engines were never really competitive in the context of the purchasing power of the average early post-war British aeromodelling enthusiast. There's little doubt in my mind that the quality of the engines fully justified their high prices, but manufacturers could only get away with playing the pay-for-quality game if they were servicing a niche market whose participants had reason to value quality to the point where they were willing to pay a premium price for it - the Olivers are perhaps the most famous examples of manufacturers whose performance-oriented customers valued quality to such a point. 

The potential buyers for the H-P engines were not in this category - they were essentially sport fliers who just wanted a dependable engine which could be bought at an affordable price. The H-P engines fulfilled the first requirement admirably, but came nowhere near meeting the second of these criteria. Presumably significant price reductions could only be achieved through a sacrifice of quality, something which the H-P manufacturers were evidently unwilling to contemplate. 

Still, hats off to Henleyston Products for producing a range of model engines which displayed both better-than-average quality along with more than adequate performance for sport-flying use. They may not have lasted long in the marketplace, but these fine engines are definitely worth remembering!


Article © Adrian C. Duncan, Coquitlam, British Columbia, Canada

First published February 2016