Abstract: The invention provides a bypass around virtually the entire periphery of a piston in a two cycle engine without substantially increasing costs or obsoleting production tooling. This is done by reshaping a cavity formed inside an engine casting to enlarge the crank case opening and to provide three oppositely displaced bypasses. The fourth side of the piston is open to the exhaust port. Therefore, virtually the entire perimeter of the piston is bypassed to greatly increase power by improved breathing.

Abstract: The exhaust port of a spark ignition two cycle engine has the general shape of an inverted T with a base portion and a narrower upwardly extending stem portion. The top of the base portion is approximately at the level of the top of the transfer port which admits fuel-air mixture to the engine cylinder. The top of the stem is from 20.degree. to 55.degree. of crankshaft rotation in advance of the opening of the transfer port. This configuration of the exhaust port produces a low broad blowdown pulse which in turn results in a low noise level of the exhaust.