Pontiac engines / blocks that have 3 inch mains are 326 / 350 / 389 / 400. Keep in mind that when I say this, it means the cid of the original block casting. Pontiac engines / blocks that have 3.25 inch mains are the 428 and 455 blocks. The most common main journal size for aftermarket blocks is 3"; however, 3.25" mains are also available as well. Now for those who are into measuring things and have the crankshaft available to them, the 3 inch main blocks have a crank sealing surface of approximately 3.188 inches. This would require the TIP-RM300 seal The 3.25 inch main bearing cranks have a sealing surface of 3.436 which would use the TIP-RM325.
So when ordering your seal, know what main bearing journal size you have. If it is a 3" main you will need a TIP-RM300. If it's a 3.25" rear main journal, then it is a TIP-RM325.
One question we get asked frequently is, "What rear main seal do I need to order?"
