Crankshaft rotation structure for four cycle engine

摘要

A crankshaft rotation structure for a four cycle engine has a radial ball bearing fitted into a first main bearing section of a crankcase and a radial roller bearing fitted into a second main bearing section of the crankcase. The crankshaft is rotatably supported by the radial ball bearing and radial roller bearing. Furthermore, an oil jet nozzle for cooling a piston is provided in the second main bearing section. The diameter of the shafts for attachment of the bearings can be made the same, even if the outer diameter of the radial roller bearing is decreased compared to the outer diameter of the radial ball bearing. Accordingly, even if the oil jet nozzle is provided in the second main bearing section into which the radial roller bearing is fitted, the distance from the crankshaft center to the attachment position of the oil jet can be decreased and the engine itself can be reduced in size.