Piston pins are made of hardened steel alloy, and their surfaces are precision finished. Piston sleeve bearings or bushings are made of bronze or a similar material. These pins and pin bear-ings require very little service and total failure seldom occurs.

Wear, pitting, and scoring are the usual troubles encountered with piston pins and piston pin bearings.

Wear of a pin or bearing is normal, but the rate of wear can be unnecessarily increased by such factors as inadequate and improper lubrica-tion, overloading, misalignment of parts, or failure of adjacent parts.

Every time a piston assembly is removed from an engine, the complete assembly should be checked for wear. Piston pins and bushings should be measured with a micrometer to determine if wear is excessive. Do NOT measure areas that do not make contact, such as those between the con-necting rod and piston bosses, and the areas under the oil holes and grooves. The correct and limiting values for measurements may be found in the manufacturers technical manual for the par-ticular engine.

Excessive wear of pins, bushings, or bearings is often the result of insufficient or improper lubrication. (These parts are usually pressure lubricated.) The failure of a pressure lubricating system is usually detected before piston pins, bushings, or bearings are seriously damaged.

Insufficient lubrication of these parts is usually caused by obstructions blocking the oil passages of the connecting rods. If the bushings have been installed so that the oil holes do not line up, lubrication may be restricted. Such misalignment of oil holes may also be caused by a bushing com-ing loose and revolving slightly out of position.

Also interchanging the upper and lower connect-ing rod bearings ON SOME ENGINES may obstruct the flow of oil to the upper end of the rod. Always check the manufacturers technical manual for information on interchangeability of parts.

If there is misalignment of the connecting rods, uneven loading on piston pins and bearings will result. The fact that a rod is misaligned is usually indicated by uneven wear of the piston pin and bushing and by piston skirt wear. Misalign-ment may be caused by improper reaming of the bushing for proper clearance.