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Hi Eric, the low compression engines, for sidecar work originally, were a fairly brief thing, in the 1927/8 era, and confined to short-stroke motors. It was done by fitting pistons with a lower crown height. Most original Flyer/Rep/Sprint pistons have a capital R stamped on the crown, but the low comp pistons have a T stamped on them, presumably for “Touring”. My 1927 long wheelbase Flyer had them fitted, and it was a delightful engine, unbelievably tractable, flexible, and smooth. It would pull away in top gear from a walking pace, without a trace of four-stroking or hesitation.
There is no difference in barrel height between shortstroke and longstroke engines. It is just the cranks and conrods that alter the stroke. ALL two-speeder production engines are shortstroke. S/S barrels have four inlet ports each side, and L/S barrels have six. The first production L/S engines were the RZ/RY TT Replica engines introduced at the end of 1928, and based on the 1928 works Isle-of-Man TT engines.