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Engine Rebuilding: The Crankshaft

Installation tips and procedures

In Engine Rebuilding: The Preparation, we discussed various ways to rebuild your engine and how to find a competent machine shop to help you in the process. Part 2 will deal with the first step in putting it all back together—installing the crankshaft in the cylinder block.

Getting Ready

When you get your engine parts back from the machine shop everything should be ready for a final cleaning and assembly. Make sure that you have everything you need before you start the process. That will include the cylinder block, crankshaft, main bearings, rear main seal, torque wrench and Plastigauge. If you are fully equipped with inside and outside micrometers, you can skip the Plastigauge.

The first step is to thoroughly wash the engine block with detergent and water for the last time and blow it dry with compressed air. If you don't have compressed air it can air dry but make certain that you turn it over several times to get the water out of bolt holes, then coat all machined surfaces with a light coat of oil to prevent rust. Give the crankshaft a good scrubbing with detergent and a soft brush and a blow-dry as well. Coat the bearing surfaces with a light coat of oil.

Getting Your Bearings

If your engine is typically on its first rebuild, the engine machine shop probably ground the crankshaft .010-inch undersize to make the bearing journals round and provide a fresh bearing surface. That means that you will need to use .010 oversize (thicker) bearings to restore the correct bearing clearances. Crankshafts are ground up to .030 undersize in increments of .010. Make sure you have the correct size bearings for your crankshaft. If they have been installed in the engine, they are not returnable.

The rear main bearing seal is usually included in the gasket rebuild kit. Sometimes it is not included. Also, several vintage engines use a rope-type rear seal that is prone to leaking. A modern neoprene lip seal is usually available from aftermarket sources that specialize in your marque. So check the car club web sites and forums for availability.

Now you're finally ready to "build an engine." Flip the block upside down and carefully insert the main bearing shells into the block. The grooved half goes in the block, the smooth half fits the bearing caps. Each bearing has a locating tang on the back next to the parting edge. The correct technique is to set the bearing in place and rotate it with light pressure to engage the tang. Same for the bearing caps. Not all the bearings are the same. One main bearing is the thrust bearing and has bearing surfaces that limit the crankshaft's front-to-rear thrust motion. Check your engine manual to identify which bearing it is.

The last step before lowering the crankshaft in place is to place the rear main seal in the block and the rear main bearing cap. The lip seal should point in. If you are using a rope type seal it should be trimmed slightly above the cap surface to have some compression force on the ends. With either style, be sure to coat it with engine oil before lowering the crankshaft in place. Next, coat the bearings with commercially available engine assembly lube and carefully lower the crankshaft in place. Next take a small piece of Plastigauge and place it on top of the crankshaft bearing journal, install the bearing cap and torque to factory specifications. Then remove the bearing cap and compare the width of the Plastigauge with the scale provided. Most modern V-8s require .025 inches main-bearing clearance. Refer to your engine specs for your engine. If the clearance is okay, remove the Plastigauge and reinstall the bearing cap and torque to factory specs, doing the same on each main bearing. If the clearance doesn't match factory specs take the parts back to your machine shop. Usually, there is no problem.

When the crankshaft is completely torqued down, it should spin freely and easily by hand (unless you are using a rope-style rear main seal which has considerable drag).

Crankshaft Installation Tips

> Make sure you have everything before you start—block, crank, bearings, rear main seal, Plastigauge, and torque wrench.

> Check that you have the correct oversize bearings if the crankshaft has been ground undersize. The bearing size is usually stamped on the backside of the bearing.

> The block and crank can never be too clean. One speck of dirt can score your freshly ground crankshaft and new bearing.

> Always consult the engine manual for torque specifications and bearing clearances.

> If the crankshaft doesn't spin freely when the main caps are torqued in place, something is wrong. Take it apart and check for problems.