Fretting is the wearing away of metal between two machined surfaces not designed for movement. Several years ago we wrote about fretting between the cam gear and the camshaft. Now, according to Cummins Engine Co., M11s, ISMs and N-14 electronic engines (in the million mile range) may have a main bearing cap fretting problem. So when the time comes for rebuilding or a bearing change, pay close attention and look for fretting.
Where do you look? Cummins says fretting might occur on the main bearing cap where it seats to the engine block. This is a press or interference fit, and when the main bearing bolts are removed, the main bearing cap should remain in the block. If there is side clearance between the main bearing cap and the engine block, the engine must be removed, disassembled, a new cap installed and the engine block line bored.
On stock engines the main bearings are lasting as long as the engine and in many cases in excess of one million miles.
Fretting will remove enough metal between the main bearing caps and the engine block to eliminate the clearance between the crankshaft and main bearings. According to Cummins, fretting usually occurs on number 2, 4 and 6 main bearing caps on N-14 engines. On M11 and ISM engines, the enginemaker says fretting may occur only on number 2 and 6 main bearings.
If the main bearings are showing accelerated wear, Cummins says a product known as Plastigage should be used to check for bearing to crank clearance. Plastigage is a special extruded plastic thread used to determine the clearance in a rod or main bearing. It’s available in various diameters. PG-1 green must be used on the Cummins engines. It will check the clearance from .001 to .003 inches. The minimum clearance on a Cummins crank is .002 inches.
In order to check the clearance on number 2 main bearing, you need to remove number 1 and 3 caps. Remove the lower bearing half from each cap and keep them in order. Now you need to fabricate a shim.
To fabricate the shim, take a 1.5-inch-long piece of 4-inch schedule 40 PVC. That should be about one-quarter inch thick. Now, cut the circle in half having two pieces 180 degrees in circumference. Now, cut about an eighth or one-quarter inch off the end of each half piece. Place one of the plastic half pipes in number 1 and 3 main bearing caps. Re-install the caps into the engine and torque the main bearing bolts to only 25 ft.-lbs. This shim will hold the crankshaft up into the upper half of the main bearings.
Now, remove the oil from number 2 main bearing and cap, cut a piece of Plastigage the width of the bearing laying it across the bearing one-quarter inch off bottom dead center. Reinstall the bearing and cap then torque to 90 ft.-lbs. Then, 170 ft.-lbs. and a final torque of 255 ft.-lbs. This torque is for N-14 engines built on or before Nov. 24, 1997, and prior to engine serial number 11871545. Be certain to coat the capscrew with engine oil to obtain the proper torque without having the bolt creek.
For N-14 engines built after Nov. 24, 1997, and beginning with serial number 11871541, up to Jan. 5, 1998, and serial number 11875596, Cummins suggests the following torque specifications: 90 ft.-lbs, 170 ft.-lbs and 248 ft.-lbs. This series of N-14’s use a modified grade 8 bolt. The part number is 208346.
For engines with serial number 11875596, you must use a 12.9-grade bolt with part number 3411337. The torque specifications are as follows: 50 ft.-lbs, 100 ft.-lbs, 140 ft.-lbs. Then turn the bolt an additional 90 degrees. This is called the torque plus angle method.
Now, remove number two main bearing cap, or whichever cap you’re checking and measure the width of the Plastigage with the drawing on the envelope that the Plastigage came in. If the clearance is less than .002 inches, do not run the engine. A new main bearing cap must be installed and the block line bored.
On the M11 and ISM engines only, Cummins says Locktite 620 may be used to stop minor fretting. The mating surfaces must be cleaned and sprayed with Locktite Primer-N and allowed to dry for five minutes. The crank and main bearing saddle should be masked prior to spraying the primer. A bead of Locktite 620 around the bolt hole and on the 45-degree bevel of the main bearing cap. This cap must be installed within 15 minutes or the Locktite will be too dry. The 620 Locktite adds an additional 70 percent to the shear strength of the main bearing cap joint.
Locktite and Plastigage are available at local auto parts stores or speed shops.
Don’t forget to remove the PVC spacer out of the other two main bearing caps and reinstall the main bearings.
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The above column is the opinion of the writer and does not necessarily reflect the opinions or beliefs of Land Line Magazine.