CONNECTING RODDING

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

MEASURING THE WRISTPIN
INSPECTING THE CONNECTING ROD AND ITS BEARING

MEASURING THE WRISTPIN

A wristpin is used to link the piston and connecting rod together. A connecting rod bearing allows the rod end to rotate around the wristpin as the piston moves. Most wristpins are comprised of high quality steel, leaving them fairly oblivious to wear. To guarantee that a wristpin is not worn, you should measure its diameter with an outside micrometer(MICROMETER), and compare these measurements with the appropriate service manual specifications.

INSPECTING THE CONNECTING ROD AND ITS BEARING

The ends of the connecting rod will employ the same types of bearings as a crankshaft. Most connecting rods have a bearing at each end, one where the rod and wristpin connect, another where the rod and crankpin connect.

The visual inspection: the connecting rods in most small engines are composed of aluminum. You should visually check both the rod and its bearings for cracks and damage. If damage is found, the connecting rod is usually replaced.

A connecting rod can be of the one piece variety, or it can have two pieces, the rod itself and a connecting rod cap where it attaches to the crankshaft crankpin. A one piece connecting rod will usually use a needle bearing at each of its ends. A two piece rod often uses the aluminum rod itself as a bearing on the wristpin end, and then contain bearing inserts on its crankpin end. Bearing inserts(also referred to as plain bearings) are thin, semicircular metal shells that have bearing material bonded to their insides. An insert is usually manufactured in two halves, one which is pressed into the connecting rod cap, the other which slips into the rod end. When the rod and rod cap are bolted around the crankshaft crankpin, it is these two bearing shells that rotate against the crankpin surface. When an engine is rebuilt, such bearing shells should always be discarded and replaced.

The clearance measurement: clearance between the crankshaft crankpin and connecting rod is significant because the rod transfers compression power to the crankshaft at this point. If crankpin clearance is too large, the connecting rod end will be loose on the crankpin, allowing it to jiggle on the crankpin during engine operation. This jiggling can produce knocking, and often, because it provides an added jolt to the rod at end of each piston movement, it will bring about premature rod failure. If crankpin clearance is too small, the connection between the rod and crankpin will produce damaging friction between the parts during engine operation.

Clearance between the rod and crankpin can be measured in several ways. One is to measure the outside diameter of the crankpin, and the inside diameter of the connecting rod and rod cap assembly. The difference between the two measurements would give you the crankpin clearance. Another way to measure crankpin clearance is with gauging plastic called Plastigage.