1981 Johnson/Evinrude 4HP Outboards Service Manual, Page 42Get this manual
Since the pistons, with the piston rings, receive the force of combustion in the cylinder head, it is necessary that both the pistons and piston rings be properly fitted to form seal between the piston head and cylinder wallsTo retain maximum power within the cylinder head, the cylinder must be perfectly round and the piston rings correctly seated in their groovesCONNECTING RODS, CRANKSHAFT, AND BEARINGS The connecting rods provide linkage between the piston and crankshaftThe force of com-
bustion, applied to the piston in reciprocating straight line thrust, is converted to rotating power at the crankshaft through the linkage of the connecting rodThe crankshaft is of the two-throw type and is supported by two cast-in main bearings and an upper needle bearingCOOLING SYSTEM The powerhead is water cooledA water pump, driven by the driveshaft, forces water through channels in the powerheadHeated water is discharged through the exhaust tube