How to Read GM Rod Bearings

by Don Bowman
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Most GM vehicles have a preset rod bearing clearance prerequisite of .001 to.003 thousandths. Bearings are offered in oversize increments to compensate for worn or inaccurate sizing of the journals as well as the fitment of the connecting rods. Rods tend to expand over time and should be resized if standard bearings are to be used. A worn or loose rod bearing will destroy the crankshaft journal it is riding on, emit a knocking sound and reduce oil pressure.

Step 1

Rotate the crankshaft with a socket on the crankshaft bolt to bring each journal to bottom center to remove and replace the rod caps. This also allows room to use the micrometer on the journal. Remove the connecting rod cap bolts using the ½-inch socket and ratchet.

Step 2

Remove the cap if it will pull off; if not, tap the side of the cap gently with a small hammer to coax it along. Wipe the bearing off with a clean cloth and observe the surface of the bearing. Some scuffing is normal and anticipated. Inordinate scuffing to the point that the bearing surface has been removed, resulting in a copper appearance, is severe wear. Take the bearing out of the cap and hold it up. Look at the overall thickness. If it is worn in the center or on the outside corners, the bearing is ruined. The bad bearing is a result of a problem, which must now be determined.

Step 3

Wipe off the crankshaft journal and look at it closely. If there is any evident scoring or grooves that can be felt as a fingernail is dragged across the surface, this is the problem. There is nothing that can be done to correct this except removing the crankshaft and having it machined or replacing it altogether. Installing new bearings will not work because the crank will eat or in other words destroy the bearings immediately. If the surface is smooth with no noticeable grooves, the journal should be checked for a perfect circle. If all the journals are clean as described and perfectly round, new bearings are needed. If not, the crank is no good, period.

Step 4

Call a machine shop to find out what the journal diameter is for the particular engine being inspected. Another place to get the dimensions is in a engine repair manual that can be purchased at many auto parts stores. Place the micrometer on the journal and tighten the adjuster to the point that the micrometer can still be rotated. Slowly rotate the micrometer from one side of the journal to the other side in a threading motion. If the micrometer hangs up or has resistance at any point in its rotation, the crankshaft journal is not perfectly round and needs to be replaced. If the crankshaft is more than .003 thousandths smaller than its original dimensions, it is damaged. The journals on new crankshafts are treated with "Tuff Ride," a tempering technique, to make them resistant to scratches. Once the tempering wears off, the crank journal will scratch very quickly due to sediment in the oil.

Step 5

Order and install a new set of "standard" rod bearings if the crank is good and the journal is within limits in diameter. If the crank is good and the bearings are not worn unevenly but are loose, check the clearance to determine the size bearings (oversized) needed. To do this calculation, one good bearing, both top and bottom, will be needed. If all the bearings showed considerable wear, one new standard bearing must be purchased to check the clearance, unless you are very good with the micrometer. Use the micrometer to check the exact measurements down to .010 thousandths at three spots on the same journal. Compare this reading with the original size and the amount of wear will be evident giving you your oversize to compensate for the wear and still have between .001 and .003 clearance. If the crankshaft journal was uneven, a rudimentary interpolation will be needed.t

Step 6

Add the three undersized differences and divide by three for an average under size for the crankshaft, and this will be the amount of oversize needed on the bearings to compensate for the uneven undersizing of the crank. If this is method is difficult for you or you do not trust this figure, it is inexpensive to buy just one bearing for this test. This test will guarantee the right bearing size and serve the same purpose for checking the crankshaft journal size.

Step 7

Perform this test on all rod bearing journals. Wipe the lower bearing in the cap clean of all oil with a clean rag. Tear off a half-inch of Plastigauge and pull the very thin hair-like blue string out of the paper housing. Place the thin piece on the top of the bearing running width-wise. Carefully install the connecting rod cap and torque the nuts down to the prescribed torque. If you are unfamiliar with the torque on the engine, then it would be wise to invest in a engine maintenance manual. They need to have an exact torque. This differs in all engines.

Step 8

Remove the nuts and the bearing cap. Hold the piece of paper the Plastigauge came in with the width measurement comparisons on the side next to the now flattened string of Plastigauge. Compare the thickness of the flattened string with the different widths on the Plastigauge housing paper. When both the flattened Plastigauge and the paper measurements agree, look at the number above and it will state how loose the bearing is. You want to be between .001 and .003 thousandths, so if the number states .004, you are .001 to loose. By installing a .002 oversize bearing you will be in the middle at .002 clearance which is fine.

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