Main bearing cap fretting is attributed to the failure of the main bearing capscrews to provide sufficient clamp load for extreme duty cycles. Fretting at the block and main bearing cap interface results in an oval shape and an overall decrease in the main bearing bore size. This decrease reduces the oil film thickness, which can lead to spun main bearings. Fretting typically occurs on the number 2, 3, 5, or 6 main bearing caps; therefore, inspection of the number 2, 3, 5, and 6 main bearing caps only is sufficient.
If fretting on any of the main bearing caps is occurring, this inspection procedure must be performed. Before performing the bore inspection, determine if the bearings are reusable. Refer to Procedure 001-006 in Section 1. Determine if there is side-to-side press fit of the main cap in the block. If there is, proceed with the inspection. If there is not, a side-to-side press fit, a service cap must be used. If a service cap is required, the inspection procedure is not required and the engine will need to be removed from the chassis and prepared for line boring.
The inspection must be performed with the main cap bearings that will be used in the engine after the inspection and/or repair is completed.
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. There 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 rages. Both Plastigage® thread and its matching envelope have a distinctive color for each clearance range.
This condemnation limit was developed to make sure of the proper oil film thickness between the main bearings and crankshaft.
Minimum N14 Series Engines Condemnation Limit:
Block Not Condemned
The main bearing bore passes inspection and can be reassembled with new standard bearing shells if all the following conditions exist:
For clearance greater than or equal to 0.051 mm [0.002 in], reassemble the main caps. Use the torque specification in use at the time the engine was built.
The main bearing bore does not pass inspection if the following conditions exist:
Repair options for blocks that have been condemned:
*Reference the Alternative Repair Manual, Bulletin 3379035, for line boring procedures.
NOTE: Cummins Inc. must approve all warrantable block repairs.