Name PMC IMP Designer Moore (and Baily)
Type Compression Ignition Capacity 0.75cc
Bore 0.314" Stroke 0.488"
Production run unknown Country of Origin England
Photos by Ken Croft and
Bert Streigler
Year of manufacture 1980



Peter Chinn's Latest Engine News column in the Aeromodeller of March, 1968 announced the release of the first new English diesel for some appreciable time [1]. The makers were given as Messrs. Moore & Baily of Groby, Leicestershire; Chinn observing that entirely new British engines were "...somewhat rare these days". The following month, PGF followed up with more detail, highlighting an unusual feature of the engine which was supplied with three spacing rings of different depth (7/64, 1/8, and 9/64"). These permitted the user to vary the position of the cylinder ports in relation to the crankcase axis, effectively altering the timing of the engine—a unique and curious feature [2].

The "EmBee 75" (the makers initials) was a nicely made little engine. The crankcase was machined from barstock with the front shaft bushing glued into the main case section with Araldite (a well known English two part epoxy). The finish was excellent and the engines were well-behaved and easy to start. Chinn suggested that it "[would] fill a long felt want for a Mills .75 replacement engine". I was unable to locate any advertising for the EmBee, but Clanford's A-Z, not always the most reliable reference, indicates that production spanned the period 1968 through 1970 [3].

A variation on the engine re-surfaced in a single advertisement appearing in the October 1980 issue of the Aeromodeller. The ancestry is obvious; the most significant change being the transposition of the inlet and the exhaust. And proving that it always pays to read the fine print, note that the add states Prototype engine photo only. Now called the "PMC IMP", the capacity was unchanged and although no manufacturer was stated, we now know it was being produced by Peter Moore on the Isle of Mann.


Actual engines, when they appeared, lacked the fine finish and attention to detail evident on the previous engine. The beam mounting lugs had been replaced by a bulkhead mounting ring. The cooling head shape was different. The front bushing was still glued in place, but was reduced in length so that the crankshaft featured sufficient end-play to ram it against the inside face of the backplate. The distance involved was more than enough to permit the conrod to fall off the crankpin with the shaft at its forward position! The engine pictured above has been restored by Motor Boy, Ken Croft.

The engine at the head of this page belongs to Bert Streigler. On it, the bulkhead mounting plate has been dispensed with—presumably because 1-1/2" aluminium bar stock is expensive—and the engine was mounted from the aft side of the bulkhead, thus clamping the backplate in place!

Internally, the cylinder ports are drilled so as to leave a land at the rear of the engine in the region where the fully floating 3/32" wrist pin would ride (not unusual and a sound feature). In a move probably unique in model engine history, the liner carries a pair of spare exhaust ports at the front of the engine—the only exhaust exit in the case being at the rear. I'll go out on a limb and say this is because the liners were left-over EmBee stock as the ancestor engine had twin side facing exhausts. Rotating an EmBee liner through 90° would produce the configuration seen in the Imp.

The transfer passage is formed by filing a flat on the OD of the liner from skirt to just above the transfer passages. These were positioned opposite the inlet, making for a somewhat unusual scavenging pattern. Although the geometry no longer required a land between the inlet ports for wrist pin retention, the twin hole design was retained, as was the "variable timing" ring, although it was now a press fit on the liner.

Lastly, note the crescent cut-out in the liner skirt on the transfer side only. With so much else about the engine indicating poor quality control, we were initially quick to ascribe this to be a provision for conrod clearance on that side only due to asymmetry in the crankcase manufacture (ie, unintentional desaxé). But then Ken realized that when the transfer flat is filed/machined, the conical relief in the lower liner skirt must result in a cut-out appearing on that side. So this is not as bad as it looks!

The piston crown is unusual too, carrying two deflector baffle cut-outs: a sloping one for transfer, and a stepped one for exhaust! The retention of the pip left from parting off the piston blank is either a highly advanced turbulence inducing device, a stop to prevent over-compression, or an indicator of the quality control process being employed. I'm going with the latter .

The purpose of the bevelled areas on the top of the case, evident on both engines, is not readily apparent. The venturi and other components are comparatively standard. The crankshaft makes no attempt at static balance, but this is not unusual in engines of this size. Now, saving the biggest surprise for last: does it run, and if so how well? Remarkably, they seem to run quite well and produce quite sufficient power for their intended purpose as sport free-flight engines. Sure proves something—not sure what...


The engine appeared Downunder in the June 1981 issue of Airborne advertised as a "hand-built" engine (to me, implying quality and attention to detail), available as plain or ball bearing. The illustration is still the prototype engine which used a slightly modified EmBee crankcase. I wonder how many eager collectors placed orders on the strength of this add and how they felt about their hand-built engines when the arrived. But it runs well, and is not ugly. Built with more (sorry 'bout that) attention to detail and finish, the IMP would be a fine little engine and is eminently suitable for home constructors. We have produced full CAD plans for the engine. These are available free to Members. Non-members can purchase them for $US15, postage paid; contact us for ordering details. Examples built from the MEN plans appear in the Gallery.


[1] Chinn, PGF: Latest Engine News, Aeromodeller, Volume XXXIII, Number 386, March 1968, p154.
[2] Chinn, PGF: Latest Engine News, Aeromodeller, Volume XXXIII, Number 387, April 1968, p202.
[3] Clanford, M: A Pictorial A To Z of Vintage and Classic Model Airplane Engines, Clan Enterprises, Surrey 1987, p63.




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