Short Stroke Engine

Under Construction!

Last year the development was mostly head and chassis. This year's major surgery is a new short stroke crank and the cams. This task should not be underestimated. Almost no engine component remains unchanged.

I have taken the decision that I am not prepared to move the head/barrel/crankcase studs to accommodate an enormous bore. Careful measurement suggests that a bore of 90mm might be possible, but I prefer to opt for 86mm, which just happens to be the diameter of two spare standard Ducati 900SS bevel drive pistons that I have on the shelf. These are not too domed and have big valve pockets. (At the moment the compression ratio, more than 10:1 is too high, still pinking under load even with octane booster) 

The 86mm bore coupled with an 86mm stroke happens to keep the bike at the 500cc mark, just like a Velocette!. A 14% reduction in stroke should give me more than the revs I am seeking.

Barrel

I have acquired an un-machined alloy casting of a Model 50 barrel. The Model 50 stroke is 88mm, leaving a convenient margine for finishing. Research shows that cost is not a factor when deciding whether to use a steel sleeve for the bore or to chrome the alloy internal working surface. So, in order to leave as much material as possible for the spiggot that will protrude down into the crankcase mouth, and to reduce dead weight, I will go with the all alloy solution.

            Left to Right...

The lack of differential expansion with alloy barrels and alloy piston means that the clearance can be run much finer, something like 0.0015"

Differential expansion does however need to be considered when designing the squish band as there is an alloy barrel and steel con rod. The squish would increase as temperature rises. This is the opposite of the Commando situation with alloy rods and steel barrels.