How to Check for Bad Main Bearings

by Chris Stevenson
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The automotive internal combustion engine has a crankshaft, rods and pistons as main components. The rods and crankshaft rotate reciprocally on protective bearing sleeves that provide a frictionless seal between the metal surfaces. Main bearing sleeves come in two half-moon sections, one that sits inside the main bearing cap and one that sits in the engine block. The crankshaft journal spins within the two. The main bearings have oil holes that lubricate the journal. Main bearings can wear with normal age, or become pitted and galled with insufficient lubrication. Checking for main bad bearings requires a process of elimination.

Step 1

Place the vehicle in park or neutral, depending upon your transmission type. Set the emergency brake and raise the hood. Do not start the engine. Pull the oil dipstick and use a rag to wipe it clean. Reinsert it and pull it up. Move the dipstick into sunlight and check for any metal reflections in the oil. Metal shavings in the oil with be the first indication of main bearing material that has sloughed off, although it could originate from the rod bearings, cam bearings or rings.

Step 2

Check the "Full" oil level line on the dipstick. If low, add enough oil to bring it to full capacity. Start the engine and let it reach normal operating temperature. Look at your oil pressure gauge, or oil warning light, if so equipped. Refer to your owner's manual for the correct oil pressure in pounds per square inch for your vehicle.

Step 3

Raise the engine's rpm to a fast idle. If the oil pressure reads below normal, this could indicate a problem with the main bearings, barring any defect with the oil quality and oil filter. It could also indicate a rod bearing that has spun on the journal.

Step 4

Turn the engine off. Hook up the positive and negative leads of a timing light to positive and negative terminals on your battery. Clip the timing light plug lead onto your no. 1 cylinder. Refer to your owner's manual for its location.

Step 5

Start the engine and point the timing down toward the crankshaft. Listen for any heavy knock or clunk coming from the engine. If you have a rod or bearing knock, the timing light will flash twice for every knocking sound. If it flashes once for every knock, you have a valve train problem.

Step 6

Turn the engine off. Use a floor jack to lift the front of the vehicle and place two jack stands under the frame. Lift the rear of the vehicle and place two jack stands under the rear frame. Slide under the vehicle and don an automotive stethoscope.

Step 7

Instruct an assistant to start your engine. Place the stethoscope pad against the bottom of the oil pan and listen for any clunking or knocking sounds. Have your assistant rev the engine a few time to increase the load. A knock at this location indicates a rod or main bearing failure.

Step 8

Use a socket and wrench to remove the bolts holding any heat shield or skid plate underneath the oil pan. Remove the shield. If you have a structural cross-member in the way, use a socket to remove the bolts and pull the cross-member down. Remove any other part that impedes access to the oil pan cover. Use a socket to remove the oil drain plug and let the oil drain into a pan. Use a socket and extension to loosen all of the oil pan bolts. Pull the pan free.

Step 9

Loosen and remove the through bolts on the engine motor mounts with a socket if you do not have sufficient clearance to pull the oil pan out. Place the floor jack under the bell housing or the crankshaft damper to lift the engine up 3 to 4 inches for clearance---no higher. Remove the oil pan.

Step 10

Fit a socket and breaker bar on a main bearing cap bolt and loosen it a few turns. Switch to the other bolt on the same bearing cap and loosen it a few turns. Alternate loosening both bolts until you have removed them. Pull the main bearing cap off the crankshaft journal and inspect the bearing sleeve. The sleeve should not have any deep grooves, pits or marred discoloration.

Step 11

Wipe the crankshaft journal and bearing cap off with a rag. Place a strip of plastigauge inside the bearing cap, according to the kit instructions. Place the bearing cap back over the crankshaft journal and screw the main bearing cap bolts in by hand. Refer to your owner's repair manual for the proper torque, in foot-pounds, required for the main bearings. Use a torque wrench to tighten the main cap bolts, but do so in increments, alternating between each bolt to get an even tightening pressure on the cap.

Step 12

Loosen the main bearing cap bolts with a socket and pull the bearing cap off. Use the kit gauge to measure the crushed thickness of the plastigauge. Refer to your owner's manual for the proper thickness allowed, indicated in thousandths of a inch. Worn main bearings will show a lower-than-normal thickness reading and must be replaced.

Step 13

Check all of your main bearings in this fashion, one at a time. Record all of the measurements and compare them with your specification limits. Replace any obviously worn bearings, or those that do not crush to the proper thickness according to the plastigauge kit procedure. Do not forget to re-torque the bearings' caps when reinstalling the old bearings or replacing them with new ones.

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