Pontiac engines / blocks that have 3 inch mains are 326 / 350 / 389 / 400. Keep in mind that when we say this, it means the cid of the original block casting. Pontiac engines / blocks that have 3.25 inch mains are the 421, 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 handy, the 3 inch main crankshaft will have a crank sealing surface of approximately 3.188 inches. This would require the TIP-RM300 seal. The 3.25 inch main bearing crankshafts have a sealing surface of approximately 3.436 which would use the TIP-RM325.
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. Another thing that we did at Tin Indian Performance is to color code our seals. Our 3″ seal, TIP-RM300 is purple in color while our 3.25″ TIP-RM325 seal is black. Just a quick way to know / make sure you that have the right seal for your application.