BORE & STROKE

Regarding the letter from Tony Coleman in RC202, questioning Frank’s usage of the classification of an engine’s bore and stroke relationship. The classification method is not the work of Frank Westworth, it is an international classification for all internal combustion engines, regardless of size. So regardless of whether the engine is 50cc, 500cc, 5000cc, or 15000cc the classification remains the same.

An engine with a bore dimension that is larger than the stroke dimension is always a short-stroke engine. An engine with a bore that is smaller than the stroke is always a long-stroke engine. Where the bore dimension is equal to the stroke, this is always a ‘square’ engine. This is regardless of the engine manufacturer or country of origin, and it can be changed on an engine by just changing the bore size and retaining the same crankshaft.

A perfect example would be the twin cylinder Hondas of the 1960s. My 1965 Honda 250cc CB72 has a bore of 54mm and a stroke 54mm, so is classified as a square engine. The 1965 Honda 305cc CB77 has a 60mm bore and a 54mm stroke, so it is classed as a short-stroke version of the engine. Both engines internally are identical, apart from the size of the bore. British and European manufacturers did exactly the same thing as a cheap and easy way of increasing the size of an engine already in production.

Whenever an engine’s dimensions are written down on paper, it is always written bore x stroke, to avoid confusion. I hope that this clarifies the situation as any other method of classification would prove confusing. Bob Livesey, member 7257 Thanks for the clarification, Bob. Just to further fuel the fire I found this lovely little illustration… with the displacement measured in litres and the bore / stroke in inches. I’d better not be put in charge of sending any spaceships to Mars… Rowena