INSPECTING THE CONNECTING ROD AND ITS BEARING

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

THE CONNECTING ROD
INSPECTING THE CONNECTING ROD AND ITS BEARING

THE CONNECTING ROD

A connecting rod is used to connect the piston to the crankshaft. One end of the connecting rod is attached to the hollow inside of the piston by a wristpin. The wristpin passes through both the piston and top of the connecting rod.

Connecting rods may be either two piece(called split), or one piece. In the case of a split style rod, its bottom, where it surrounds the crankpin(CRANKSHAFT), splits apart for installation and removal. The end which detaches from the main part of the rod is called the connecting rod cap. In a small engine, when the rod is installed on the crankpin, the connecting rod cap will usually be secured to the main rod by just two bolts.

When piston and connecting rod are joined, the piston fits over the top part of the rod. The wristpin slides through a hole in the piston, then passes through the top end of the rod and is secured by two spring steel clips that fit into machined recesses in the hole of the piston.

INSPECTING THE CONNECTING ROD AND ITS BEARING

The ends of a connecting rod employ the same types of bearings as the crankshaft. Usually a connecting rod possesses a bearing at each end, one where it connects to wristpin, another where it connects to the crankpin.

The visual inspection: connecting rods in most small engines are composed of aluminum. You should examine them for cracks and other damage to determine if they can be reused. If damage is found, the rod will almost always need to be replaced.

A one piece rod will usually contain needle bearings at each end. On the other hand, a two piece rod will either use its own aluminum to function as the crankpin bearing, or contain bearing inserts. Bearing inserts(also called plain bearings) are thin, semicircular metal shells that have bearing material bonded to their insides. Like the two piece rod itself, they are manufactured in two separate pieces, one which is pushed into the rod cap, the other into connecting rod end. When the rod and cap are clamped around the crankpin, these bearing shell inserts become, in essence, a cylindrical bearing that covers the crankpin surface for easier crankshaft rotation. When an engine is rebuilt, these bearing shell inserts should be discarded and replaced.

The clearance measurement: proper clearance between the crankpin and connecting rod end is important because this is the point of power transfer to the crankshaft. If the clearance here is too large, the rod end will be loose on the crankpin, allowing the rod to move up and down on the crankpin during engine operation. Such movement may produce knocking, and abrupt jolts to the connecting rod at the end of each piston stroke will eventually cause it to fail. If the clearance between rod and crankpin is too small, during engine operation excessive friction will build up between the parts, causing damage and premature wear.

Clearance between the rod and crankpin can be measured in several ways. One way is simply to measure the outside diameter of crankpin and the inside diameter of the connecting rod end, and compare their differences to the service manual specification for acceptable crankpin clearance. Another way to measure crankpin clearance is to use a gauging plastic called Plastigage(SEE SOMETHING).