Pepperell Seven-Sixteenth


Name Seven-Sixteenths Designer Vern Pepperell
Type Compression Ignition Capacity 1.56cc
Production run Lost Country of Origin New Zealand
Photo by David Owen Year of manufacture 1946-ish



This engine is a good representative of the Pepperell fixed-jet, compression ignition engines manufactured by Vern and Ira Pepperell in New Zealand from 1946 to 1953 (spark ignition Pepperells extend back to 1936). The "diesel" concept originated on The Continent and was introduced to the rest of the world by modeller soldiers returning from that theatre following World War II. In New Zealand, it was Bob McQuillan who showed Vern Pepperell his Italian Antares 4cc. Vern loosely based his first designs on the Anthares. These were side-port, with in-piston transfer valves like the Ray Arden 1940 Atom and 1938 M&M. Disappointed with the power output, the in-piston transfer port was replaced by a front rotary valve, multiple internal transfer passages, and 360 degree exhaust porting. Vern was granted New Zealand provisional patent No 95137 for his design in 1945 [1].

Our review sample is almost identical to the "Godwit Trophy" Seven-Sixteenths engine pictured on page 70 of Those Incredible Pepperells [1]. The text states that this particular engine had the top two fins carefully filed back by some previous owner. Either this was a common procedure, or some engines were made like this as David's engine lacks the fins, but does not appear to have been modified. In fact, the only apparent difference between David's engine and the Godwit Trophy version is in the air intake, with the latter having a single, forward-facing inlet, while David's example is transverse-drilled, resulting in two side-facing intakes.

The most unusual feature of the "Peps" was the absence of adjustable fuel metering. Vern believed that an engine with one less thing for an inexperienced operator to meddle with would be easier to operate, so he followed the practice he'd seen on the Arden Atom and used a fine, fixed fuel jet, with an adjustable air supply. The jet was a stub, drilled almost all the way through, then pricked the last distance with a darning needle and a hammer! The resulting jet (about 0.015" or so) worked fine provided owners took the trouble to filter their fuel. The Jet was a screw fit and could be removed for blowing out if it became blocked. The rotating air-tap is a taper fit in the venturi with light spring pressure applied by the little alclad tab holding it in place and providing enough friction to hopefully prevent it moving under vibration. The taper was produced using the front part of a hand-reamer--all of which goes to show that the Pepperells were very pragmatic people!

The crankshaft was turned from chrome molly steel salvaged from Morris axels which Ira reported "..broke like carrots.." in New Zealand, providing a ready, cheap material supply. Only the crankpin was hardened obviating any need for grinding to re-true the shaft. The shaft is a mighty 0.375" diameter and the thread is the US standard: 1/4-28. The cylinder was turned from nickle-steel and heat-blued, but un-hardened. Exhaust porting was 360 degree and transfer effected by drilling eight 1/8" holes around the bore PCD before boring. These terminated about 1/32" of an inch below the exhaust ports. The top of the cast iron piston was in the form of a truncated cone to assist deflection of the transfer gasses. The solid contra-piston had a similar depression to match the piston crown. The very low-set wrist pin was held captive (permanently, see photo) by soft solder! This appears to have been done before final honing of the piston. Pistons were individually fitted to liners. The conrod was milled and filed from steel and case-hardened. It has a single step on its forward face to clear the crescent counter-balance weight on the crank web. This design assures that the piston would always be reassembled in the correct orientation (it is a very bad thing to reverse the orientation of a piston on a run-in engine).

The crankcases were gravity die-cast by the Pepperells from alloy salvaged from aircraft parts. An interview with Vern in Reference [1] explains that the castings were not cored and describes the early tribulations they experienced with porous castings before hitting on an appropriate de-gassing compound for the alloy they were using. All machine cut threads on the Pepperells were 40 TPI. This was chosen so they would be shallow and could be cut in one pass (not something I'd try!), followed by "chasing". The crankshaft ran in the reamed crankcase; no bushing was employed. Obviously the die must have split laterally to release the casting, but almost all trace of the parting line in the case has been filed and polished away. It is not known if this was a "factory" feature, or the results of some previous owner's beautification project.

The remainder of the components are shown here. The inner and outer backplates seal with paper-like gaskets. The 40 TPI thread is run right down to the sholder, indicating that a very narrow tool was used, at least at the leading edge of the 60 degree V. The inevitable tiny gap at the sholder was compensated for by relieving the female thread by a turn or three at the crankcase openings. Notice that the outer plate bears the NZ patent number (though precisely what was patented is not known, the Patent Office original having been destroyed by fire). The plain faced prop drive washer is a bit odd. The slight raised circle around the keyed hole that mates with a square section on the crankshaft appears to have been sawed off, leading me to question if the little drive-peg is original. Certainly none of the Peps in Ref [1] have such a feature. The rear of the drive washer is a bit butchered in the area of the peg, increasing my unease over its originality.

As mentioned earlier, the air-value is very slightly tapered, corresponding to the lead taper of a hand-reamer used to open out the valve seat in the crankcase. Air valves were drilled on assembly so that they would be in the fully open position when the arm was resting against the right hand side of the cylinder. The tank cap screw (the brass doo-dad in the lower middle of the picture) is not original, but David believes it is an accurate reproduction. Like the comp-screw, it is 10-32 and has a light V milled longitudinally up the thread to act as an air-vent. Neat.

What impressed me the most about the engine was that despite the somewhat crude appearance and very rudimentary equipment used to produce it (in one bedroom of the very modest Pepperell family home), all fits and finish of running parts are of a very high standard. The "dare to be different" design features and pragmatic approach to problems (like wrist-pin retention) indicate a refreshing degree of origanility. In fact, I like the engine so much that CAD drawings (7 sheets) have been produced and a reproduction will be made soon (or soon-ish, don't hold your breath ). Thanks must go to David Owen for being brave enough to apply sufficient heat to his rare treasure to enable disassembly and measurement almost 60 years after it was assembled.


[1] Poletti, M: Those Incredible Pepperells, ISBN 0-473-09990-X, Deed Printing, Waiuku, New Zealand, 2003, p15.

  For more details of this book, see Editorial page July, 2004.




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