It should be noted that the Rotrax Mk2 was also available with the JAP 84S short stroke motor fitted.
The prototype (pictured*) was of Erskine design and an article in the MCN gave the following report.
An avid user of the JAP engines throughout his career, Pete Jarman, after watching the tests said, "It's the first major change to a JAP engine since 1928.
It could put the JAP right back in because the speed of the engine has never been in doubt".
Mr Erskine, who works on most of the top riders engines, has been working on the conversion since the end of the previous year. He is now to build six motors for JAP.
FRAME: Rotrax MK2 built with Reynolds tubing and low temperature bronze welded. Stove enameled silver grey. Oil tank incorporated in centre tube
FRONT FORKS: Telescopic type with concealed springs. Adjustable spring pressure.
WHEELS: Strongly built with heavy Gauge spokes and steel nipples. Knock-out spindles.
REAR SPROCKETS: Light Alloy. Sizes 56T to 62T available.
EXHAUST: Large Diameter pipe secured by two clips to frame.
FOOTREST: Solid one piece type supplied for fitting to R.H side to customer's own positional requirements.
MUDGUARDS: Front and rear in strong fibreglass.
TYRES: Special speedway 2.75 x 23 front and 350 x 19 rear. Two solid security bolts fitted to rear wheel.
CLUTCH: Modified Speedway type with steel plates gear-tooth cut to fit clutch centre. Chain wheel 42T. Final drive sprocket 15T. Also available 14T, 16T and 17T.
COUNTERSHAFT: Hardened steel shaft on ball bearings in alloy case.
ENGINE: 499cc JAP 4 valve DOHC 84mm bore x 90mm stroke for use on methanol fuel only.
CARBURETTOR: Amal concentric type, 34mm bore 12.5 downdraft. Modified for Methanol fuels fitted with gauze air cleaner and operated by very quick action twistgrip.
MAGNETO: Electronic Magneto.
* One or more images missing (×)