Fretting is micro-motion between the main bearing cap and block that results in material loss at the joint. This motion causes a loss of crankshaft to main bearing running clearance that can lead to a spun main bearing, a spun connecting rod bearing, and/or a broken crankshaft.
There is risk when overhauling or replacing the main bearings in a block that has incurred main cap fretting. Inserting new standard main bearing shells into a joint that has sustained sufficient loss of material can lead to zero crankshaft/bearing clearance. This can cause the crankshaft to lock in place.
Recent long-hour engineering tests have shown the addition of Loctite®
NOTE: The inspection must be performed with the original main cap and bearing shells. During the reduction of bearing running clearance, the original main bearing develops a wear pattern, which is an indicator of main cap fretting.
Plastigage® provides a fast and accurate method to check bearing clearances. Plastigage® is a special extruded plastic thread of a definite diameter with accurately controlled crush properties. Plastigage® is packaged in calibrated envelopes, 12 envelopes per box. These envelopes not only protect the plastic threads but also serve directly as scales to measure the bearing clearance. Both sides of the envelope have a printed scale of graduations. One side is calibrated in inches, the other in millimeters. The numbers on the scale are the bearing clearances in thousandths of an inch or millimeter. When the width of a compressed section of Plastigage® in a bearing or journal is compared with the appropriate-numbered graduation, the bearing clearance can be read directly from the scale.
Plastigage® is available, from a local parts store, in four styles to cover different clearance ranges. Both Plastigage® thread and its matching envelope have a distinctive color for each clearance range.
Condemnation limits were developed to make sure of proper oil film thickness between the main bearings and crankshaft. The minimum acceptable clearance is 0.051 mm [.002 in] for the ISM series engines. Blocks not
The block is not condemned if the main bearing bore passes inspection and can be reassembled with new standard bearing shells if all of the following conditions exist:
The block is condemned if the main bearing bore does not
Repair options for blocks that have been condemned:
NOTE: Cummins must approve all warrantable block repairs.