Here’s a better description of several points that I made earlier in this thread on how to rebuild a Pontiac V-8. Some of you asked how to check thrust bearing clearance. Others wanted to know about the potential for rear main seal leaks due to machining errors.
This first picture shows how to check thrust bearing clearance using a feeler gauge. The concept is the same no matter how you do it; you need to rock the crank back and forth to make sure you have enough, but not too much, free play in the crank. Measure between the face of the bearing insert and the crank. In case of Pontiac V-8s, you are looking for .003 to .009 thrust bearing clearance. On Pontiac motors, the thrust bearing is installed in the #4 main bearing bore (not all V-8s are the same.)
Here's another way to check thrust bearing clearance. This method is a little more precise. What you need to do is set up a dial gauge that is zeroed out when the crank is pressed all the way towards the back of the block. When you pry (gently) the crank back and forth, you can directly read the clearance value off the dial gauge.
This picture shows the machine work around the rear main. You can see the groove where the rear main seal fits. In some cases, the factory didn't get the machine work perfectly concentric around the crank, and I’ll explain why this can cause a problem.
In this picture, you can see the use of a vernier caliper to measure the depth of the rear main seal groove on the driver’s side of the block. The next picture shows the same measurement on the passenger’s side of the block.
If the rear main seal groove was not concentric around the center line of the crank, the two readings will be different. Apparently when the factory installed the original asbestos style rear main seals back in the 60s and 70s this was not a problem. With the newer style seals, the ability to compensate for variations in machine work isn’t as good, and this problem can lead to a nasty leak. My machinist says he has only seen this error on 455 blocks. Nonetheless, once burned, twice shy, so he always checks when working on a Pontiac motor.