The piston of an internal combustion engine is subject to many exacting requirements which must be fulfilled to produce the modern automobile. It must move rapidly and smoothly up and down in the cylinder bore with as little friction as possible while efficiently transferring the explosion impulses to the connecting rods and crankshaft. Efficient engine performance is dependent upon the ability of the piston to keep the explosion gases out of the crankcase and the crankcase lubricating oil out of the combustion chamber. The piston must be able to disperse the heat picked up during each power impulse so that excessive piston temperatures will not build up to cause pre-ignition. Its design and expansivity must be such that as it and the cylinder become hot, the clearance between them is maintained so that there is no excessive pressure between piston and cylinder causing seizure or scoring, and when the engine is cold this clearance must be small enough to prevent slapping. Furthermore, it must weigh as little as possible so that vibrational forces due to rapid reciprocating motion are at a minimum. Vibration as a result of heavy pistons is a factor in engines in which the inertia forces are unbalanced such as in four-cylinder engines.