OMC Stern Drives And Motors 1964-1986 Repair Manual., Page 36Get this manual
ENGINE GENERAL PRINCIPLES
Engine specification charts are located in the AppendixThese charts can be used determine the engine type, size, and specificationsAll engines used to power inboard-outboard boats, except some of the two-cycle engines operate on the four-stroke cycle principleDuring this cycle, the piston travels the length of its stroke four timesAs the piston travels up and down, the crankshaft is rotated half way (180 degrees)To complete one full cycle, the crankshaft rotates two complete turns; the camshaft, which controls the valves, is driven by the crankshaft half crankshaft speedValve action, intake and exhaust, occurs once in each four-stroke cycle, and the piston acts as an air pump during the two remaining strokes