The engine of the bike in the photograph is in my workshop now. When John Catchpole had it, he added weight to the flywheel, as I do in principle but using a different method. he made a special pair of cranks of heavier construction with a crankpin of 7/8″ diameter. Naturally this needed unique crankpin bushes with thinner than usual section. Rods are lightened and even had aluminium little end bushes. Inlet tract is polished out a bit and the inlet port bridges thinned.
Basically this incorporates all the traditional tuning modifications of the period, and is very well done. After Catchpole, it was owned by Derek Shire who used it to good effect in sprinting. For this purpose, the flywheel was reduced in diameter and the engine fitted with a TT Amac carb modified with a second jet to use alcohol and amyl acetate fuel. The barrel had been bored to top oversize and +60 short stroke pistons fitted. The bores and pistons were worn out so I skimmed a minimum out of the bores and have made a pair of pistons from castings in LM14. These turned out to be lighter than the previous short stroke items. The flywheel has had weight added again and the engine will have caged big ends and shimmed small ends. I have been a bit of a maverick all my life and prefer to think out my own solutions to problems, so my upgrades incorporate several non traditional methods, mostly to give a more free passage to the inlet.
With the blessing of the owner, this engine will have some of my thinking added to that of John Catchpole and Derek Shire. I feel privileged to be able to help ensure that this historic bike is kept on the road and retains its sporting character. I intend to write a piece for Yowl in the near future
