Pepperell diesel test

The Pepperell Seven-Sixteenths and Half-Inch Engines

By Maris Dislers

The few years following WW2 saw an unprecedented explosion of activity worldwide in the field of model engine production. For a while, demand was so high that anyone with a bit of nous and access to even fairly rudimentary machinery could get into the engine making business. Of course there was plenty of chaff among the wheat, but also some outstandingly talented constructors. I’d unhesitatingly include New Zealanders Vern and Ira Pepperell in the latter group. In my view, their achievements deserve far wider recognition than they have been generally accorded.

In large part, the relative obscurity of the Pepperell engines is down to the fact that very little readily-accessible documentation on them has hitherto been available. In particular, I’m not aware of any published performance tests (aside from a claimed power output of 0.29 BHP at 11,500 RPM  in the 1950 Pep 21-B diesel’s instructions). I happen to be the lucky owner of a Pepperell Half-Inch compression ignition engine in good nick, so I feel obliged to report on what it can do.

The Pepperells’ varied productions are covered in detail by Maurice Poletti in his book entitled "Those Incredible Pepperells" (ISBN 0-437-09990-X). This book is now long out of print, so I’ll go over the “history” bit briefly for context. This material is taken with due acknowledgement from Maurice’s book.

Setting the Scene

Vern Pepperell was a resident of Onehunga, a suburb of Auckland lying some 8 kilometers south of the Auckland city centre on New Zealand's North Island. He had a solid engineering background in the motor trade and manufacturing fields, while his son Ira was the keen aeromodeller who pestered dear old Dad into making him an engine for his models in 1936. This was somewhat like a side exhaust Brown Junior. The two had some notable successes with this engine until WW2 got in the way. Ira joined the Royal New Zealand Air Force (RNZAF) at this point in time.

Model engines were unobtainable during the war, but Vern anticipated the supply shortage once peace was declared. The New Zealand government didn’t disappoint, instituting a restrictive post-war import licensing policy that made engines from the USA especially hard to obtain at a time when supplies from other British Commonwealth countries were scant. This situation favoured home-grown products, leading Vern to set up in a very modest way to manufacture his “Godwit” miniature petrol (spark ignition) engines. Hardly salubrious, with two lathes, a pedestal drill and bench grinder housed inside the family home and a simple oil-burning furnace out back for casting crankcases.

The Godwits came in two displacements, having half-inch bore/stroke and five-eighths inch bore/stroke dimensions for displacements of 098 cuin. !.6 cc)  and .192 cuin. (3.15 cc) respectively. They utilized disc rear rotary valve (RRV) induction and a valve-in-piston (VIP) transfer system, which was inspired by the American M&M “Piston Valve Motor” of 1939 and emulated by the Japanese TOP engines. Around two dozen of each of these Godwit units were made.

There were also a few far larger engines of similar type with .79 and .86 cuin. (12.95 cc and 14.10 cc) displacements, which were made to special order, and a one-off .38 cuin. (6.29 cc) geared flat twin using two .19 cylinder assemblies. At around this time, Ira left the RNZAF with a wealth of aircraft engineering knowledge and skills, joining his dad in their home workshop at Onehunga.

As of the early post-WW2 years, compression ignition was THE latest thing in model engine technology – the commercial miniature glow-plug didn’t arrive to joint the diesel in challenging the established sparkers until late 1947. However, there was relatively little design innovation at this time - new diesel constructors everywhere more or less followed the established European design trends which had been evolving since 1941 from a few influential models such as the Swiss Dyno-1.

Not so Vern Pepperell, who saw the obvious potential of compression ignition technology and quickly developed a design for such a model, being granted New Zealand Provisional Patent No. 95137 for this design in 1945. He followed this up by making an experimental  2.1 cc diesel in early 1946. Vern’s first diesel was very broadly based on an Italian Antares 4 cc engine which had been brought back from Europe by fellow Auckland Model Aero Club (AMAC) member Bob McQuillan. However, being an ideas man with a fertile imagination and an experimental bent, Vern was not content merely to copy – his rendition incorporated the Godwit VIP transfer method, making it quite unique among diesels. It ran well, but fell short of achieving the anticipated power output.

Without further waste of time, a “teaser” advert in the May 1946 issue of Whites Aviation magazine alerted readers to watch for the forthcoming “Compression Ignition Godwits”. The June 1946 issue included a photo and details of the pre-production “Pepperell Half Inch” diesel. This featured crankshaft front rotary valve (FRV) induction, radial mounting, an on-board fuel tank within the hollow rear crankcase cover, half-inch cylinder bore and (presumably) Arden-style 360 degree porting. The prototype engine was reported to weigh 4 ounces and produce 1/7 H.P. using a fuel mix of 55 parts kerosene, 30 parts lubrication oil and 15 parts ether. Note that the pictured prototype in the attached Whites Aviation photo is incomplete - wot, no needle valve?!?

After a few more changes, production examples soon followed, being advertised in Whites Aviation magazine for September 1946 by Wisemans of 32 Customs Street, Auckland for £9-15-0 and £10-15-0 (displacements unspecified). These were almost certainly the seven-sixteenth inch and half-inch bore types that were to become the volume Pepperell sellers. Note that these were pretty steep prices by the standards of the day, but people would save up and buy them anyway because the Government import licensing policies pretty much eliminated any overseas competition at the time. The engines were unique in being identified by their bore dimensions rather than by their displacements.

If these two models are obscure now, the other Pepperell engines are barely remembered at all. Fortunately, Maurice’s book shows many other Pepperell engines, the majority of which were experimental types. His CAD drawings of these models were sometimes necessarily based on fuzzy photos, production drawings and the memories of Ira and others who used them at the time. They include various diesels on the original theme from 0.037 cuin. through 0.23 cuin. all the way up to 0.455 cuin. There were also derivative in-line twins and glowplug types.

In around 1948, Vern developed a keen interest in tether cars, making potent .61, .40 and .19 cuin. racing engines to suit this application. These were apparently excellent performers. If the Pepperell .61 in the accompanying image from Maurice Poletti's book reminds you of a Dooling, the similarity is no coincidence! Vern Pepperell and Tom Dooling corresponded regularly, sharing each other’s developments and ideas. Typically, only one or perhaps a few examples of each of these racing models were made, although a castings/plan set was offered for the .61.

Around 20 or 30 of the “scaled down” .037 cuin. (0.61 cc) Baby diesel units were made in 1948, and in 1950 the 5 cc Pepperell-Tetley 29 glow-plug model appeared, looking rather like a K&B Torpedo 29. A somewhat larger number of 1950 Pep 21-B 3.5 cc diesels were produced, along with a handful of the smaller companion 0.1 cuin. (1.62 cc) Pep 10 diesels. The latter model was a very neat and up-to-date design indeed by 1950 standards.

Changing market conditions eventually caused a shift in focus to making Pepperell glow plugs and Whirlwind wood propellers. However, the economic return wasn’t there, ultimately leading Ira to pursue work in full size aviation, while Vern switched to making packaging machinery. Series manufacture of the Pepperell engines ceased in 1952. If you want more detail, hunt down a copy of Maurice’s book (good luck!).    

Pepperell Engine Design

The first generation Pepperells share an easily-identified style. Vern Pepperell’s combination of Ray Arden’s avant-garde design features with European compression ignition technology into his Godwit diesels was well ahead of the times in 1946. Reason enough for some members of the now-defunct Motor Boys International model enginering group to make replicas of the late David Owen’s original Pepperell Seven-Sixteenths engine no. 47237. That’s code for (reading backwards) the 37th engine made in February 1947. Ken Croft's image of his superb replica rendition of this engine is reproduced at the right.

The replica builders followed the late Ron Chernich’s meticulously accurate drawings of the original, using crankcase castings produced by Eric Offen. Ron's General Arrangement drawing is reproduced below. Read more about it on the “Model Engine News” website. This is my major information source together with Maurice’s book and my own Half-inch engine no. 48679.  

Some of the specification values in Maurice’s book seem incorrect. These are probably typos, but thankfully his scale 3-view CAD drawings appear to be accurate. Together with what we know from David’s Seven-Sixteenths and my Half-inch, it is most likely that the two original production Pepperells shared a common crankcase and most other components, differing only in bore size.

The engines were based on a very neat aluminium gravity die (permold) cast crankcase, using recycled scrap aircraft components to supply the raw material. Originally the cases were provided with  three reinforcing webs between main case and crankshaft bearing portions, but they were soon modified to have a generous radius at the transition, overcoming the potential for stress cracks to develop.

The  unhardened chrome molybdenum steel crankshaft was typically made from easily-obtained broken Morris car axles. Ira Pepperell later recalled that the ready availability of these components was down to the fact that they "broke like carrots" under the tough driving conditions encountered in New Zealand!

The crankshaft featured a generous .375 in. dia. main journal along with a cyanide-hardened 0.219 in. dia. crankpin and a crescent counterbalance with or without cut-away crankweb flanks. The main journal included a circular rotary intake port for FRV induction. The shaft included a square section at the front to key the aluminium prop driver.

At the rear, a screw-in primary backplate sealed the crankcase, with a larger secondary backplate sealing the rear fuel tank space which formed part of the primary backplate. The engines featured two-point radial mounting. The accompanying rear view of Ken Croft's lovely Seven-Sixteenths replica shows these features very clearly.

The primary backplate is located in the usual position, threading into the crankcase in the normal manner. The secondary backplate which closes off the primary backplate cavity to form the fuel tank space is clearly visible in the accompanying view. The threaded brass filler hole cover has a fine slot to admit air while discouraging dirt from reaching the fuel metering jet.

The engines utilized a hardened steel conrod having a rectangular section, offset to clear the crankshaft counterweight. A cast iron piston was employed, with a conical crown and low-set wrist pin location. The cast iron contra piston had a matching conical combustion chamber shape. The unhardened blued steel cylinder was threaded both above the exhausts to accommodate a finned aluminium cooling jacket (muff) and below the cylinder location flange for assembly into the crankcase.

Another instance of Vern Pepperell’s original thinking is to be found in the carburettor design. Vern chose a fixed jet/variable choke carburettor rather than the usual fixed choke/variable jet type, reasoning rightly or wrongly that it simplified operation for newcomers to model diesels. This design element and the 360-degree cylinder porting were probably derived from the Arden .099 or .199 spark ignition engines of 1946, suggesting that the salient design aspects of those models were by then known in New Zealand, even if import restrictions limited their availability.

The system comprises a screw-in jet tube, partly intersecting the crankcase aperture immediately above the rotary crankshaft valve port and connected to the fuel supply. The jet aperture is a tiny hole of a size determined at the design stage to admit the correct amount of fuel. Above this sits a vertical cylindrical barrel with a circular port or ports that can be rotated to progressively close the horizontally-drilled air intake aperture(s), thereby adjusting the air supply to the engine (somewhat like a regular R/C engine throttle). The barrel is externally tapered to match the shallowly-tapered bore of the induction tube. It is retained in the induction tube by a small alclad tab which is secured with a screw threaded into a small forward extension of the induction tube at the top. This tab is formed so as to provide enough light downward spring pressure to retain the setting once established.

This arrangement allows mixture adjustment to suit the range of anticipated loads, fuels and weather conditions. It also provides a measure of speed control. Close off the air further and the engine slows down. Ideally, if the jet size is set correctly, the operator simply closes the choke (lever forward) to draw fuel up from the tank and prime the engine. Then one opens the choke (lever rearward), starts the engine and adjusts the compression setting. Simple as that!

The location and orientation of the air intake apertures varies.  Some examples, such as David Owen's Seven-Sixteenths unit, have  a pair of apertures drilled one on each side of the vertical induction tube, with a pair of holes in the air control barrel to match. Others, like my own Half-inch, have a single forward-facing aperture matched with a single port in the air control barrel.

The accompanying close-up photograph shows the fuel jet protruding into the crankcase opening for the rotary crankshaft valve. If the jet is correctly sized, no needle valve is needed with this system - the mixture strength is adjusted by varying the air supply rather than by metering the fuel supply. The opening shown in the photograph forms the effective choke area when the air control barrel is fully open.

Design Differences

The two models of the basic Pepperell design display a number of design variations. To begin with, the following table gives the major design parameters of the two models.


Pepperell Seven-Sixteenths

Pepperell Half-inch

Bore (nominal)

0.438 in (11.1 mm)

0.500 in (12.7 mm)

Stroke (nominal)

0.625 in (15.88 mm)

0.625 in (15.88 mm)

Swept Volume (nominal)

0.094 cu in (1.54 cc)

0.123 cu in (2.02 cc)

Exhaust Duration

140 degrees

116 degrees

Transfer Duration

102 degrees

104 degrees

Intake Opens

106 deg ABDC


Intake Closes

34 deg ATDC


Intake Duration

108 degrees

114 degrees

Effective Choke Area

8 sq mm (assumed)

8 sq mm


4.6 oz (130 g)

5.33 oz (151 g)

Note - Choke area measured at inlet jet position. Throttle barrel intake of half-inch is 9/64 inch (3.56 mm) diameter when fully open, giving 9.95 sq mm.

David Owen’s Seven-Sixteenths has Arden-style porting comprising eight 1/8 inch semi-circular transfer flutes (drilled before machining the bore) ending just below three narrow exhaust slots – similar to the far later Webra Mach 1 and many other similar designs. Ken Croft duplicated this design in his fine replica.

My own Half-inch (serial number 48679, making it the 79th engine made in June 1948) has only three semi-circular 1/8” transfer flutes, aligned with and terminating half way up the pillars between exhausts, giving considerable overlap between transfer and exhaust. This was a very novel design for 1948, but was soon to become quite commonly used worldwide. I do not know if this transfer porting arrangement is standard for all Half-inch engines, but the somewhat lower exhaust location needed to achieve that overlap accords with Maurice’s drawing of Half-inch engine number A48353, suggesting that mine is not unique.

The 1/8 in. dia. wrist pin in David’s Seven-Sixteenths engine is typical Pepperell. The pin is threaded at one end, being screwed into position in the piston and then retained with soft solder. My Half-inch has a regular wrist pin clamped in place by a brass rivet hammered into a small vertical hole drilled in the piston skirt on each side. Both are innovative methods, but the riveting operation had cracked my piston at the rear, such that the rivet and a chunk of iron let go in use sometime in the past. That explains the ugly score up the cylinder bore. Aside from a rather sloppy fit between the pin and the rear piston boss, the engine is otherwise in good shape.

OK, so now we know how they go together!  How does a typical example run?  To find out, I put my own Half-inch into the test stand and got stuck in.

On the Test Bench

I quickly learned the necessity of blowing out the little bits of crud that had somehow worked their way into the tiny jet aperture (around .010” or .025mm). For this system to work, everything has to be scrupulously clean. Easy enough, but the cleared jet turned out to admit too much fuel! The jet tubes were made from a standard aircraft rivet which was drilled almost through. Then the actual jet hole was punched through with a needle and adjusted by eye. I thought about peening the raised edges down a little, but trial and error could take some time to get it right. Instead, I added a remote needle valve to the fuel feed line and just got on with it. This remote needle is clearly visible in the accompanying image, just to the right of the engine in the photo.

The engine starts very easily. Having established a reasonable setting in the usual way using the remote needle valve with the air control valve fully open, it was then possible to use that valve as a rudimentary throttle. Closing the intake aperture naturally richens the mixture (faster velocity past the jet, but less air volume), eventually baulking the engine if carried too far. With the remote needle set somewhat lean at full intake opening (or the fixed jet doing that on its own in an ideal situation), a reasonable degree of speed adjustment is possible.

On the other hand, if the jet is a bit undersize (or partly blocked), requiring the air intake to be closed significantly to maintain fuel draw, the reduced effective choke area then limits peak power output. No wonder the idea was not widely adopted. Even Ray Arden subsequently abandoned it in his 1947 models in favor of the regular fixed choke/needle valve carburettor.

Response to compression adjustment was good. With smaller propellers, there was a tendency to knock when starting and the engine would then sometimes run backwards. Backing off compression for starting overcame that problem. Providing the engine is mounted sturdily, vibration levels are quite low.

I used equal parts ether, kerosene and castor oil, with 0.8% ignition improver added. On this mixture, the engine seemed to run nicely at full pelt. However, it can get quite hot when running fast, so a more heavily doped fuel would probably not be advantageous and a straight un-doped fuel would likely work just as well, especially given the fact that this is not a high-speed engine.

Performance appraisal

The engine proved to be a real pussycat with propellers down to nine inch diameter. Thereafter, the Half-inch Pep (as they were commonly called) needed ever higher compression settings to match smaller loads, with little gain resulting. On these lighter loads, the engine was simply running out of puff. Fair enough, considering the very mild cylinder port and intake durations.



APC 7x6


Cox 8x4


APC 8x4


Graupner 8x5


APC 8x6


APC 9x4


Graupner 10x3


Graupner 9x5


APC 9x6


APC 10x4


APC 10x6


APC 11x6


APC 12x6


The performance curves show the development of excellent torque upwards of 19 oz-in at 4,800 RPM, dropping by a third at the peak power point around 9,500 RPM. These characteristics resulted in the development of some 0.13 BHP at that speed. By comparison, Ken Croft’s replica Seven-Sixteenths turned a Top Flite wood 8x3.5 propeller at 11,600 RPM, which I estimate requires a somewhat higher power output despite the smaller displacement.

The results are consistent with memories of the relative performances (and popularity) of the two engine sizes. Why pay a whole pound more for very little extra performance? It was never established why the Half-inch did not deliver on its extra swept volume. The smaller bore engine easily outsold the Half-inch and remained in production right through to 1952, when Pepperell engine manufacture ceased.

Having expected a somewhat better showing from my Half-inch unit, I pondered why the performance of my example fell somewhat short of expectations. To begin with, it has very mild exhaust port timing, perhaps the result of locating the exhausts lower down to achieve the desired transfer port overlap, while drilling the transfers to the same depth as the seven-sixteenths cylinders. Could this be the design element that restricts power potential?

Out of curiosity I made a spacer to lift the cylinder by one turn of its 40 TPI thread, increasing exhaust and transfer durations to 132 degrees and 120 degrees respectively. These figures are close to what would be considered normal today. Somewhat surprisingly, spot re-tests with 10x4 9x4 and 8x4 propellers showed no change in performance.

Which brings me to the next likely performance limiter – only three transfer flutes versus the Seven-Sixteenths’ eight. Compared with the potent 1952 Webra Rekord 1.48 cc diesel (which has the same porting arrangement and a very similar bore size), the Pep’s transfers are significantly smaller in cross-section area and twice as long. Certainly more restrictive for transfer of charge to the combustion chamber as revs go up. And the rotary intake timing is also not conducive to higher speed running. Anyway, this engine is far too rare to suffer irreversible modification, so it’s time to leave well enough alone.


New Zealanders are justifiably proud that one of their own once designed and produced model engines that were at least equal to anything else then available. By comparison, the Arden .099 fell short on power output, also requiring a heavy and bulky ignition support system, until it switched to glowplug ignition in 1948. At that point, the Arden acquired a considerable advantage in size and weight - in fact, the Pepperell Half-inch had very similar external dimensions to the remarkably light and compact Arden .199. However, it must be remembered that the Pepperell was built to withstand the rough treatment which a first-time diesel user might be expected to dish out.

Among competing British products, the contemporary E.D. Mk II and Comp Special 2 cc diesels as well as the FROG 160 glow or FROG 180 Series 2 diesel couldn’t match a Pep. No wonder many New Zealanders learned to fly control line models with a Pep diesel in the nose.

Way up north in Liverpool, England, Aerol Engineering’s Frank Ellis had been independently following a very similar design approach to the Pepperells with his early Gremlin and Hurricane 2 cc diesels. His improved derivative Elfin 1.8 (released in mid 1948) could match or exceed the Pep for power and was significantly lighter and more compact. That engine probably kick-started the rapid second generation diesel development phase in the U.K. and elsewhere, which eventually pushed more than just the Pepperells out of the market. Still, a most worthy venture by a talented designer!

(Editor’s note: Before departing the model engine manufacturing field for good in 1953, Vern and Ira designed and constructed a small handful (perhaps four examples in total) of a very advanced 0.232 cuin. twin ball-race rear drum valve diesel expressly intended for Class B team racing, in which it would have to compete with the dominant ETA 29 glow-plug model produced in England by New Zealand ex-patriate Ken Bedford. Despite giving away some 0.060 cuin. displacement, this design proved to be quite competitive, being both fast and fuel-efficient. These qualities allowed it to take the first three places at the Waikato Championship Meeting for 1953 against ETA opposition. Quite a swan-song!  For full details, check out this link - A.D.).


Article © Maris Dislers, Adelaide, South Australia

Frst published February 2018