Mystery 2.5 cc diesel

An Intriguing 2.5 cc Diesel - Whodunnit?!?

This article represents a first for me in that it concerns an engine to which I can’t attach a name! This well-made but highly enigmatic 2.5 cc diesel engine was obtained from Tim Dannels in July 2017. It came to Tim in connection with his involvement in the selling off of the estate of the late Doug Wendt of Montana, USA.

Doug was a former acquaintance and trading partner of mine who started collecting engines (including diesels) way back in the 1950’s before anyone knew that there even was such a thing as engine collecting! Moreover, no-one in North America apart from Doug and a very few others (including me from 1966 onwards!) was interested in diesels back then or for some decades thereafter, leaving the likes of Doug and (later) myself with a clear field in the diesel acquisition department. As a result, Doug ended up owning many extremely rare or elusive diesels such as this one.

Unfortunately, Doug didn’t keep any detailed records of what he had, hence leaving us in complete ignorance of the source and identity of this unit. The engne itself is no help at all - it bears no marks of identification whatsoever, not even a serial number.

This said, this particular engine proved on examination to be a very well-made unit having so many interesting and potentially historically significant features that I felt it to be worth going all the way in documenting it and seeking further information from as wide a range of sources as possible, namely, my esteemed readers. It may be that one or more of its highly individual design features will trigger a memory to the effect that those features are characteristic of the work of a particular manufacturer. That would do much to narrow the focus of search.

To further this objective, the best that I can do for now is provided a detailed illustrated description of the engine together with a performance assessment. My hope is that someone who reads this article will come up with a name or at least a well-founded suggestion for the maker(s) of this obscure and seemingly mega-rare motor.

The engine’s rarity is underscored by the fact that in well over 40 years of looking I’ve never previously seen as much as a picture of an example. Given the relative rarity of side-stack diesel designs, I find this very surprising - if I'd seen it, I would certainly have taken note. As it is, I've run the engine past a number of my more knowledgeable fellow enthusiasts, several of whom claim to have "seen it before" but none of whom can put a name to it.

All I can say at present is that a Japanese origin seems highly unlikely for a variety of reasons which are covered in the following article. Everyone agrees that the engine appears to be a European product of the late 1950's. Two individuals have independently noted a few features which are characteristic of the earlier work of Hans Drenkhahn of Germany (later of Dremo fame), although there is no supporting evidence for this identification. 

To date, the most persuasive attempt at identification has been supplied by reader Robert Dulake, who drew my attention to the fact that the milled-out interior of the exhaust stack, leaving a recessed bridge in the middle, and the general unmachined surface finish of the crankcase and backplate are both more or less identical to the same features on a Super Sokol 5 cc diesel from Poland that Robert bought from Mike Clanford in the early 1980's.  My sincere thanks to Robert for this observation!

Robert's views are certainly supported by the accompanying image of a Super Sokol extracted from O. F. W. Fisher's 1977 book "Collector's Guide to Model Aero Engines". The treatment of the exhaust stack is identical to that of the mystery engine. The Super Sokol was designed by Stanislaw Gorski, Poland's most productive model engine designer who was also responsible for the well-known Jaskolka range.

The side-stack design of the Super Sokol demonstrates that Gorski was very familiar with this rather unusual design layout by diesel standards. Moreover, Gorski routinely used colour anodizing on the cooling jackets, prop drivers and spinners of his engines. It thus seemed possible, albeit unproven, that this engine was a hitherto unreported design by Gorski. 

In order to pursue this possibility, I signed up on a Polish modelling forum which included a lengthy and very interesting thread on the little-documented subject of Polish model engines. This was quite easy to do, and the use of Google's translation facility renders the site completely intelligible to an English-speaking reader like myself. I was warmly welcomed desipite my non-Polish location - in fact, I found that many Polish model engine enthusiasts are regular readers of my own website! 

However, no-one on the Polish site could identify this engine! They agree that the treatment of the exhaust stack is similar to that of the Super Sokol, but state that none of the engine's other design features correspond in any way to any of Gorski's documented design work. They state further that if this was a Polish engine, they would definitely know about it. They suggest looking further East towards Russia or perhaps the Balkan region. So although Robert's suggestion certainly had considerable merit, the identity of this engine remains  an open question - the search continues.

Wherever and by whomever it was made, the engine is remarkably elusive - it even escaped the attention of 2.5 cc guru Jim Dunkin (not a mis-spelled relation of mine!), whose wonderful book on the subject of the world's 2.5 cc engines doesn’t miss much but doesn’t include this one. I checked directly with Jim, who confirmed that he has no prior knowledge of this unit or its origins.

Engines of this rarity and obscurity often turn out to be home-builds or one-offs. In the present case, this attribution seems highly unlikely for a number of reasons. First there’s the colour anodizing, a somewhat unusual refinement in a home-build. Secondly, there are the three castings, all of which are produced as gravity die-castings. Few home builders would go to the trouble of making multiple dies for a one-off.

In addition, the engine exhibits a number of minor detail flaws which are typical of series-production models but seem to me to be unlikely to have been left unattended by any talented and self-respecting home constructor. I'll point these out as I proceed with this evaluation.

Finally, there’s the general style of construction, which of course has to be seen at first hand to be fully appreciated. On the basis of my own long and varied experience, I have to say that the manufacturing processes which have been used as well as the quality and character of machining, finish and heat-treatment are far more characteristic of a production item than a one-off.

Fair enough – we seem to have a very obscure commercial product here, or at least a prototype of a planned commercial production.  What makes it tick?  Let's have a look!


The engine in question is a 2.44 cc plain bearing crankshaft front rotary valve (FRV) diesel featuring cross-flow loop scavenging. Checked bore and stroke are 14.90 mm (0.587 in.) and 14.00 mm (0.551 in.) respectively for a displacement of 2.441 cc (0.149 cuin.). The engine weighs in at a healthy 174 gm (6.14 ounces), although I must emphasize that this includes my own steel prop mounting sleeve nut and washer which were made to replace missing originals. I have no idea whether or not they match the originals - they just look right and do the job.

I also can’t confirm the correctness of the Enya needle valve assembly which I used to replace the missing original on the basis that it was a perfect match. The Enya assembly is a good match for many engines, so this proves nothing. A different assembly could of course have some slight effect upon the engine’s weight.

We have to begin somewhere, so let’s start at the top and work down. The cooling jacket is a separate slip-on component which is turned from light alloy bar stock. It is anodized an attractive mid blue colour. It fits over the cylindrical upper portion of a cast-iron cylinder liner. It is fitted with a conventional T-bar comp screw having a M5x0.8 thread. Indeed, metric screw threads are used throughout, clearly implying an Asian or Continental origin.

The liner is vertically located by a wide flange at full cooling jacket diameter just above the cylinder ports. This is clearly visible immediately below the cooling jacket in the various images of the engine. The lower face of the cooling jacket bears against the upper surface of this flange, thus eliminating any assembly stresses from the actual bore - a very good design feature. The assembly is in fact a precise reflection of that used in the Enya D15-I, of which more below in its place. 

The three fasteners which secure the cylinder assembly pass all the way through both the cooling jacket and the cylinder location flange, thence continuing on down through clearance holes in the upper crankcase casting (see below) to engage with tapped holes in the top surface of the main (lower) crankcase casting. The entire assembly is thus radially aligned by the retaining fasteners.

Speaking of which, one of the first points of interest relates to the cylinder retaining fasteners. They are actually not bolts at all – rather, they are steel studs which are threaded M3x0.5 at both ends. What appears to be the head of each retaining fastener is actually a separate female-threaded slot-head cap which engages with the male upper threads on the studs. The upper cooling jacket holes are drilled oversize to match the caps.

The idea is clearly to thread the studs securely into the crankcase and then fit the threaded steel caps once the cylinder components are assembled on the studs. The actual stressing of the assembly is thus achieved by tightening the caps on the upper threads rather than by shifting the threads in the crankcase. This will of course do much to minimize the possibility of stripping the female crankcase threads. A quality feature in my view! Indeed, the engine gives an impression of quality throughout.

Moving on down, the next component that we encounter is the separate casting which constitutes the upper crankcase unit. This casting incorporates the exhaust stack together with a pair of internally-formed bypass passages which have been created by end milling along a common alignment. It is cast to a good standard with no visible flaws.

However, here we encounter the first of the manufacturing flaws which suggest a commercial origin - the milling of the two bypass passages is not perfectly aligned with the exhaust stack and cylinder installation holes. Although errors of this kind appear not infrequently in commercial productions due to sloppy work-setting, it's unlikely that a capable home constructor having pride in his work would get this wrong and leave it uncorrected. Having gone to all the trouble of making the necessary die, he would surely cast another component and try again.

This brings us to perhaps the most interesting feature of this engine – the bypass and transfer arrangements. The exhaust port cut into the cylinder liner is a conventional rectangular opening which feeds into a wide exhaust stack cast integrally with the upper crankcase casting. The transfer port too is a conventional rectangular feature which largely overlaps the exhaust.  

It is the manner in which this transfer port is supplied with mixture that constitutes one of the really intriguing aspects of this engine’s design. The two milled bypass passages mentioned earlier feed into the transfer port from the ends rather than in the middle in the conventional cross-flow loop scavenging arrangement. This design precisely mirrors the  arrangement introduced in late 1956 in the then-revolutionary 2.5 cc Enya D15-I diesel. The two streams of incoming mixture will logically meet head-on in the middle and be mutually deflected upwards, hence serving in effect as their own “baffle” and eliminating the need for a conventional baffle piston. 

In my possibly biased view, the D15-I represented one of the outstanding technical achievements of the Enya brothers during the “classic” era. Their style of porting using the modified form of loop scavenging described above was quite revolutionary for the time, especially in a diesel context. Indeed, it represented the first step towards the Schnuerle porting which was to become the standard for high performance engines in subsequent decades. In particular, the Enya unmistakably served as the design inspiration for the Czech MVVS 25D-1958 which was the MVVS workshop’s initial (and very successful) foray into the field of high performance diesels.

This constitutes the first major clue regarding the dating and provenance of the subject “mystery” engine. Although possible, it seems rather unlikely that this modification to the loop scavenging system was developed by our unknown manufacturer in complete isolation. Some previous acquaintance with either the Enya D15 or the MVVS 25D is clearly implied. The engine could of course be a forerunner of either model, but I have to say that I personally see this as unlikely. Either way, a Japanese, East European or perhaps Russian origin is clearly implied, as is a date of the mid to late 1950’s.

I can’t comment further on the upper cylinder assembly since I was unable to fully dismantle it using what I consider to be “fair means” thanks to the 60 year old castor gum which was gluing it together. Even moderate heat didn't shift it! As the above image shows, the style of porting leaves very little metal holding the lower cylinder liner together on the transfer side, and I wasn’t prepared to subject the cast iron cylinder liner to significant stresses in an attempt to dismantle it further.

Turning now to the piston, we once again run into a unique design feature. The piston is a composite component consisting of three elements. The working piston is a thin-walled steel shell with no bosses for the gudgeon (wrist) pin. It has a thin and relatively narrow internal flange at the top. The 6 mm dia. hollow steel gudgeon pin fits inside this shell, being located in an aluminium alloy carrier which is inserted into the piston shell to butt against the lower surface of the flange at the top.

This carrier is centred by a short spigot at the top which is a snug fit in the large circular opening at the centre of the piston shell flange. The carrier is secured by a threaded plug at full bore diameter which engages from above with a female 8 mm thread in the top of the internal carrier. This plug bears against the upper surface of the piston shell flange to form the actual piston crown, which thus has a domed shape.The flange is of course sandwiched between the two alloy components.

The upper plug is provided with a transverse slot which provides a dependable purchase to allow it to be really securely tightened. You have to hold the internal carrier against turning while doing this - don't use the con-rod!! I used a pair of circlip pliers, which worked well. Since the critical thing is to ensure that the assembly is tight at operating temperatures, I did the final tightening after heating the assembly to something approaching operating temperature. This approach proved to be completely effective, as we shall see later.

At first glance this assembly would appear to hold considerable potential for working loose during operation. However, several factors militate against this possibility. For one thing, the large outside diameter of the alloy components would create considerable frictional resistance to turning of either the carrier or upper plug against the installation flange. For another, any lateral thermal expansion of the aluminium alloy carrier would tend to further tighten the already-snug fit of its installation spigot in the centre of the installation flange. Finally, the internal carrier is exposed at all times to cool incoming mixture in the crankcase. This will result in its running considerably cooler than the upper plug which forms the piston crown, accordingly being exposed to the full heat of the combustion process. Hence the effect of operating the engine will be to expand the hot upper plug against the female thread in the cooler con-rod carrier, most likely stabilizing the threaded joint very securely.

Looking again at the cylinder, we find that the underside of the contra piston is similarly domed in a concave sense. The height of the concave dome in the underside of the contra piston is somewhat deeper than the height of the dome on the top of the composite piston. This will create a rudimentary degree of squish-induced swirl when the engine is running.

There is no intrinsic requirement to eliminate any gudgeon pin ends in the piston surface, since the ends of the pin would not traverse any portions of the cylinder wall having either transfer or exhaust ports. Indeed, the use of the separate working piston “shell” may be seen in some ways as a negative feature in that it eliminates the possibility of providing piston skirt ports to relieve the bypass bottleneck which will be discussed below. The composite construction must therefore have another overriding purpose.

It seems likely that the designer’s goals were to achieve the lightest possible piston and at the same time to minimize crankcase volume. The internal volume of the composite piston is about as small as it could be and still leave room for a con-rod, while the arrangement allows for the use of a far shorter gudgeon pin and a thinner-walled working piston, thus minimizing the amount of ferrous metal incorporated into the reciprocating components. The entire piston/gudgeon pin/conrod assembly weighs only 9 gm, a very reasonable figure for a 2.5 cc diesel.

The conrod itself is a nicely made component which has been machined from light alloy. It is basically a conventional turned “dog bone” design which has had its big end outside diameter thinned by milling. The unbushed bearings at both ends have an internal diameter of 6 mm. They are very well fitted to their respective journals.

Arriving at the main crankcase at long last, we find a few more points of interest. First, while the casting seems sound enough with no visible porosity, it is not without a flaw. Note the incomplete left-hand mounting lug which evidently resulted from the metal solidifying before the die was completely filled. I doubt very much that any self-respecting home constructor would have let that pass - having taken the trouble to make a die, he'd simply have cast another case.

The general style of the casting is one of the factors which argues against a Japanese origin. By the mid to late 1950's, pressure die-casting as opposed to gravity die-casting was the order of the day in Japan. Moreover, the standard of finish on Japanese castings was generally higher than seen in the subject mystery engine.

Returning to our description, although only three studs are used to secure the cylinder assembly in this engine, the upper flange of the main crankcase has a ring of six tapped holes drilled in it The purpose of this is clearly to allow the upper cylinder assembly to be oriented with the exhaust stack to either the right (as illustrated) or the left. A thoughtful touch!

A comment which is far more pertinent to the engine’s functional design is the matter of supply of mixture from the main crankcase to the bypass and transfer passages. The bypass passages formed in the separate upper crankcase casting are not duplicated in the lower casting - the presence of the alternative assembly holes precludes this. Nor can piston skirt ports be provided due to the impossibility of guaranteeing the working piston shell’s orientation when assembled. As a result, when the piston is at or near bottom dead centre (BDC), the only path for gas access to each upper bypass passage is through a short annular passage bounded in cross-section by the ends of the slots in the cylinder wall below the bypass, the piston wall and the inner crankcase wall respectively. 

The dimensions of this slot are 1.35 mm wide (the thickness of the lower cylinder wall) by approximately 6.5 mm annular length. Each annular slot formed in this way has an area of roughly 9 mm2 for a somewhat marginal combined total for the two bypass passages of 18 mm2. Below the lower cylinder wall, the bypass consists of a 360 degree 1.35 mm wide annular space between the piston wall and the inner wall of the crankcase.

It’s clear that although they are very short in terms of vertical length, these two annular sections of the bypass passage immediately above the base of the cylinder liner will constitute the governing constriction for gas flow from the lower crankcase to the actual transfer ports. The situation could be greatly eased by cutting what would amount to extensions of the upper bypass passages into the lower crankcase - there's enough metal there to accommodate this. However, in the interests of conservation I have no intention of applying such a modification to this very rare engine.

Yet another argument in favour of this engine being a production unit as opposed to a home-construction effort may be derived from the fact that the lower bypass cutaways in the cylinder liner do not precisely correspond to the bypass passages milled into the upper casting. Once again, I have great difficulty seeing any self-respecting home constructor failing to pay attention to this detail, particularly since the matching of the liner cutaways would in fact involve their widening, with a consequent easing of the previously-noted bypass constrictions which they represent. 

The motor’s cylinder port timing figures are fairly conventional. The exhaust opens at 112 degrees after top dead centre (ATDC) for a total exhaust period of 136 degrees. The transfer opens some 8 degrees later for a total transfer period of 120 degrees.  

Taking a further look at the main crankcase casting, we find that the integrally-cast main bearing section has a bronze bushing inserted in it. This bushing has a rectangular induction window cut through it which registers with the base of the circular intake tract in the main casting. This design will increase the rapidity with which the induction system opens and closes.

The casting incorporates an undrilled boss beneath the intake to allow for the potential use of valved crankcase pressure fuel feed. This implies that a high performance glow-plug version may have been contemplated when the crankcase die was created.

When I first saw images of this engine, I assumed that it must be a twin ball-race design given the very large diameter of the main bearing housing. I was therefore extremely surprised to find on first-hand examination that it is in fact a plain bearing unit. The reason for the large-diameter main bearing housing becomes apparent when we examine the one-piece hardened steel crankshaft. This has a truly monumental main journal diameter of no less than 12 mm – the largest that I have ever encountered in an engine of this displacement!

The very well-made and finely finished crankshaft is more or less conventional apart from its unusually large main journal diameter. It is a truly superb fit in its bronze bushing. The very high quality of the surface finishes on the journal and crankpin suggest the possession of high-class grinding equipment along with a good knowledge of how to use such equipment to best advantage. Once again, this implies the involvement of a commercial manufacturer.

The crankdisc is generously counterbalanced, leading one to expect relatively low levels of vibration when combined with the very light reciprocating components. The central gas passage is 6.5 mm in diameter, leaving a large amount of surrounding metal to ensure adequate strength.

The induction port in the shaft is perfectly round - another Enya-like feature. This is a good shape for strength given the absence of sharp corners to act as stress-raisers. Its diameter of 8 mm provides an adequate area when fully open. An unusual feature of the shaft assembly is the inclusion of a 0.2 mm thick steel thrust washer between the front of the crankdisc and the rear of the bronze bearing insert.

The massive dimensions of the shaft in this engine may possibly be explained by a limited availability of suitable steel for this highly-stressed component. However, the general similarity of of this shaft design to that of the Enya may actually represent another straw in the wind implying direct Enya influence. The original Enya D15-I had a shaft journal diameter of "only" 10 mm, but over time it acquired a certain reputation for shaft breakages. An observant designer who was following the Enya design pattern might well respond to this situation by increasing the shaft diameter. Enya themselves certainly did so - their D15-II of 1961 continued the same cylinder porting arrangements as our subject stranger but also featured an almost equally massive main journal diameter of 11.5 mm. If the documented problems with the Enya D15-I influenced our unknown designer's choice of a main journal diameter, he was in very good company!

The induction port timing is again fairly conventional. The induction port opens more or less concurrently with the closure of the transfer port at 60 degrees after bottom dead centre (ABDC), while the system closes at 45 degrees ATDC. This provides a total induction period of 165 degrees. There is no supplementary sub-piston induction.  

The alloy prop driver is anodized blue to match the cooling jacket. It is located on a taper formed at the front of the main journal. A relatively short integrally-machined externally-threaded extension protrudes from the centre of the shaft to allow for prop mounting. The length of this is insufficient to permit the use of a conventional nut and washer for securing the prop, making it clear that the engine originally had a sleeve nut of some kind which is now missing. I made up an Oliver-style steel sleeve nut and washer combination, which are seen in the images. The thread is M6x0.75. However, I'd hazard a guess that the original component may well have been an aluminium alloy spinner nut with an extended hub.

In addition to the prop nut, the engine was also missing its needle valve assembly as received. The needle valve installation hole is 4 mm in diameter, which exactly matches a standard Enya spraybar. Moreover, the jet on an Enya item aligns very precisely with the axial centre line of the intake, making that assembly a perfect match. Accordingly, I fitted an Enya assembly to the engine, while recognizing that there’s no evidence whatsoever that this assembly is anywhere near correct.

The separate plug-in venturi has an internal diameter of 7 mm. The application of Maris Dislers’ very useful Excel calculator confirms that in combination with the 4 mm diameter spraybar, this provides an effective choke area of 12.093 mm2. According to Maris’s calculator, this is a very useable set-up for free flight or control line performance applications where maximum power is the goal rather than optimal suction. The calculator tells us that the combination should work OK provided the engine speed is kept above 9,917 RPM. Since we would expect an engine exhibiting these design features to operate at a considerably higher speed than that, this combination of spraybar and venturi diameters should work fine for this engine.

The final component to be described is the backplate. This is another gravity die-casting in light alloy. It has a small protrusion at one edge of its internal recess between two of the mounting holes, which normally indicates an internal shelf to provide piston skirt clearance. However, there’s no such shelf in this instance – the internal installation spigot is a perfect cylinder. It appears that the manufacturer(s) may have contemplated the use of this casting in a different design which did require such clearance.

The backplate is secured using four countersunk head 3 mm machine screws – another quality touch. Countersunk screws create a very elegant appearance due to their heads being flush with the component being secured. They also have a greatly reduced tendency to work loose due to the wedging action of the tapered countersink. However, they do impose considerable wedge-induced bursting stresses upon the surrounding material which can result in cracking of the eyelets in the casting if too much force is used. Snug but not too tight is the condition to aim for!

Overall, the standards of machining, fitting and finish exhibited by this engine are very high - we're dealing with a quality product here. However, the presence of the manufacturing flaws noted above argues strongly against a Japanese origin - no Japanese manufacturer wishing to compete with the likes of the Enya brothers would allow such errors to appear in his products.

The motor appears to have seen considerable use in the past, but all bearings remain very closely fitted. The only operational issue resulting from extensive previous use is a certain softening of the compression seal, although by no means enough to create insurmountable starting or running difficulties. Whoever built this engine was a capable designer who knew what he was doing and had the equipment to do it! 

OK, now you know as much as I do about this engine’s design and construction features. Let’s see how it runs!  

The Mystery Engine Struts its Stuff

Heading into the testing phase of this study, I wasn't expecting our mysterious subject to deliver any mind-blowing levels of performance. The very large-diameter shaft would create a higher-than-usual amount of friction and viscous drag, while the previously-discussed constriction of the bypass passages would unquestionably have a negative effect upon the engine's breathing, particularly at the higher speeds. 

However, none of this would prevent the motor from developing quite reasonable torque figures at lower operating speeds. This being the case, I elected to begin with a 10x6 Taipan airscrew - no doubt a far larger prop than anyone would have actually used with this unit for flying purposes.

Once in the test stand with the 10x6 prop fitted, it became immediately apparent that the piston/cylinder fit was definitely on the "soft" side as a result of the seemingly considerable amount of use which the engine had clearly received from previous owners. Accordingly, I used an oily fuel containing 30% castor oil to help the seal during operation. I also added a small oil prime to the exhaust prime which the engine turned out to prefer for starting. Subsequently starts demonstrated that this latter refinement was not absolutely necessary, although it did help starting to some extent.

Following the administration of a composite prime in this manner, the engine commenced firing immediately and started up readily enough. I'd set the needle at 3 turns open, which turned out to be a good cold starting setting, albeit a little on the rich side for best running. The Enya needle valve assembly proved to be a perfect match for the engine, providing very precise mixture control. The contra piston was if anything a little too tightly fitted, but it remained adjustable at all times.

Once set correctly, running qualities were beyond reproach. The exhaust note was crisp and clean with no trace of a misfire. Despite the fact that it was a very hot and somewhat humid day, the engine ran very steadly with no tendency to sag as the run went on. The exhaust residues were nice and clear, with little visible smoke. As expected, vibration levels were extremely low.

Response to the controls was exceptionally good. The needle response in particular was particulary conducive to the easy establishment of the optimum setting. Go a little too lean and a slight "crackle" developed in the exhaust note, although the engine kept running steadily without slowing down. Go a little rich, and the engine slowed down slightly while continuing to run very smoothly with no misfiring and minimal smoke. I had to open the needle quite a long way to produce the usual smoky "burp-burp" running with which all diesel users are familiar. As a result, the needle setting was very far from critical. The best needle setting was of course just before the leaned-out "crackle" first appeared, hence being very easily established - lean out to the crackle and then back off just a little.

As I expected, the engine showed good torque development at the lower speeds. As the prop load was reduced and speeds accordingly climbed, the engine continued to start readily and run flawlessly. The figures obtained on the tested airscrews were as follows, along with the derived power curve:

10x6 Taipan 8,000 0.181
10x4 Taipan 9,600 0.226
9x6 APC 9,900 0.234
8x6 APC 11,700 0.279
71/2x4 APC WB 12,900 0.290
8x4 APC 13,700 0.280







The 71/2x4 APC WB is a cut-down APC 9x4 created and calibrated to fill a gap in the torque absorption figure for my test set of props. As the power curve clearly shows, it filled a critical gap in the present instance - without it, I'd have missed the peak!

I have to say that the above figures exceeded my expectations going in. The implied peak output of around 0.290 BHP @ 12,900 RPM was achieved using the aforementioned APC wide-blade prop. I suspect that the engine could do somewhat better than this on a less opressively hot 'n humid day (not a good combination for best power output) and with a more closely-fitted piston. Under more ideal conditions, I truly believe that 0.300 BHP @ 13,500 RPM is not beyond the bounds of possibility.

Even so, the figures obtained under the less-than-perfect test conditions are actually pretty good going for a plain bearing 2.5 cc diesel of (presumably) late 1950's vintage. The engine doesn't fall far short of the reported outputs of either the single ball-race Enya D15-I or the plain bearing MVVS 25D-1958 upon which its transfer porting appears to have been modelled. Peter Chinn's reported figures for those two designs were 0.298 BHP @ 14,700 RPM and 0.295 BHP @ 15,700 RPM respectively.

Obviously, both the Enya and the MVVS developed their peak outputs at significantly higher RPM than our mysterious test subject, which clearly develops superior mid-range torque to either of the other units. I have no doubt at all that the difference in peaking speeds is largely down to the two previously-noted constrictions in the bypass passages of the test example, which seem to cause the engine to begin running out of breath above 13,000 RPM. Friction losses from the very large main crankshaft jounal doubtless also made a contribution.

Those bypass constrictions could easily be eliminated by creating a downward extension of each bypass passage in the lower crankcase casting - there's plenty of metal to accommodate this. If such a modification were made, I could easily see the mystery engine's peaking speed being increased by as much as 1000 RPM with a consequent improvement in its already quite impressive peak output. However, I'm leaving well alone - I have no plans to fly this engine, besides which a unit of this rarity and technical interest should be preserved as-is. In any case, its performance as it stands really isn't half bad! 

The engine came through its testing with flying colours, showing no signs of any mechanical distress at any time. I was careful to keep checking the integrity of the composite piston assembly as the testing proceeded, both visually through the exhaust port and by feel. It was gratifying to find that the assembly remained completely stable throughout. I would be quite comfortable using the engine with this assembly installed. 

All in all, a very satisfactory test! This fascinating engine is a credit to its unknown designer and manufacturer.  I only wish that we could identify those responible to give them the acknowledgement that they richly deserve!


This intriguing powerplant displays a sufficient range of unusual technical features that its identification and dating are matters having some priority in the context of the history of model diesel development during the "classic" era. In particular, its use of a very similar bypass/transfer arrangement to that seen in the Enya and MVVS 2.5 cc diesels of the mid to late 1950's raises the question of its relationship to those designs. Was it a precursor, an influence, a later copy or a simple case of parallel devleopment? Only a secure date and an authoritative identification of its source could answer those questions.

At present, I'd have to say that the very clear evidence in terms of possible Enya or MVVS influence seems to date this engine to the latter half of the 1950's. The question of the engine's geographic origin is another matter. The style of the castings is not late 1950's Japanese - it has more of the character of German, Eastern European or perhaps Russian work as far as I'm concerned.

So my present best guess (which is all that I can offer at present) is that this is a late 1950's product from Germany, Eastern Europe or perhaps the USSR (as it was then). It may represent a proposed production series that never got off the ground. Robert Dulake's suggestion of a Polish origin with Stanislaw Gorski as the designer seemed completely credible, but failed to stand up under scrutiny. 

I've spent far more time and energy than usual on what is actually just another Wotizit because as stated earlier I personally believe it to be important in the context of the development history of model diesels that we secure an authoritative identification and date for this one. This being the case, I'll close with a plea for anyone who can shed light on this fascinating design to please get in touch, either directly or through the blog site! Any and all input will be very sincerely appreciated as well as being openly and gratefully achnowledged!


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

First published September 2017