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How to Set Bearing Clearances When Engine Rebuilding

When rebuilding an engine, the clearance between the main bearings and the crankshafts main bearing journals are critical. Too much clearance results in engine knocks, while not enough clearance results in burned bearings because they cannot get proper lubrication. These clearances are especially important to measure and adjust after machining the crankshaft to remove imperfections. Although it sounds complicated, measuring the clearances and adjusting them to specifications is really quite simple.

Instructions

1

Dip a set of standard size main bearings in engine oil and slip them into the block and main bearing caps.

2

Set the crankshaft in the block, ensuring that the main bearing journals of the crankshaft sit on the main bearings in the block.

3

Smear a light coating of grease across the center of the bearing journals on the crankshaft. Put a string of Plastigauge in the grease on each main bearing journal. The grease will keep the Plastigauge in place.

4

Install each bearing cap in its original position, thread the bearing cap bolts into the block through the bearing caps. Tighten the bearing caps with any tools necessary from a toolkit. Torque the main bearing cap bolts with a torque wrench.

5

Remove the main bearing caps, one bearing cap at a time. Measure the Plastigauge with the built in scale on the Plastigauge packaging. Subtract the recommended factory main bearing clearance from the Plastigauge reading. The result will be the size of the oversize bearing needed for that particular main bearing journal. Repeat for all main bearing caps.

6

Remove the crankshaft. Slip the appropriate over-sized bearing in the block required for each crankshaft journal, depending on their individual Plastigauge readings, after dipping them in motor oil. Slide the other half of the bearing into the main bearing cap after dipping it in motor oil. Plastigauge each bearing again to ensure all bearing clearances are correct before proceeding any further. Adjust the bearing sizes as needed.