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Which rear main seal do I need for my Pontiac engine

One question we get asked frequently is, “Which rear main seal do I need to order for my 1964 – 1979 Pontiac engine?”

 

Pontiac rear main seal TIP-RM300 seal lip and logo
TIP-RM300 3″ Viton ® Pontiac rear main seal lip

 

At Tin Indian Performance, we sell / advertise our Viton ® rear main seals according to the main journal size of the appropriate engine that uses them.  The TIP-RM300 seal is designed to be used in 3″ main journal blocks and the TIP-RM325 seal is designed for the 3.25″ main blocks.

 

Pontiac rear main seal TIP-M300 installed in stock cap
TIP-RM300 3″ Viton ® Pontiac rear main seal TIP-M300 installed in stock cap

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.

Pontiac rear main seal TIP-RM300 OD of seal
TIP-RM300 3″ Viton ® rear main seal OD of seal

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.