[Wade Congdon]

At BOP Engineering we get a lot of calls and emails regarding our rear main seals for the early pre-'59 Pontiacs. Here is all the info you need to choose the right seal.

You have several choices for a Viton® seal for your pre '59 Pontiac V-8 (287, 316, 347, 370) however both cases require modification. The 3-inch main seals (RMS01/RMS18 one-piece & two-piece respectively) will fit you groove perfectly but have the wrong diameter.

The 2-piece can be cut down and reformed. This has been done many times over the past 20 years and worked well, although it is difficult to reform as the 2-piece has a stiff metal insert.

The 1-piece seal has a more flexible core and is cut at a witness mark where the core is split. At this point it looks like a lock washer. This cut normally ('59 and newer) remains unchanged during the fitting procedure. The seal is fit to the block by trying it in the block/cap without the crank in and removing material on the OD of the seal if necessary to complete the fit by getting the lips to come together with minimal distortion.

Because the pre '59 crank and smaller you can "cut" the ends back further effectively making the seal smaller. This is usually done on a belt sander or disc sander removing material from the cut end(s) effectively removing a section from the seal. Like the regular installation, it requires trying the seal in the block and cap to complete the fit. Hence the fitting procedure is the same, you just remove material in a different location. We have many customers doing this with the big block Buicks using the 3.25-inch Pontiac main seal.

So the 1-piece seal is easier to fit to the early engines due to it's flexible core and gives you the advantage of a double lip sealing surface. Note that the '59 389 is the first "modern" seal configuration and oil pan rail configuration and therfore is a direct fit with the BOP rear main seals and one-piece oil pan gaskets. Hope this answers your question and as always call anytime with questions.

[Bill Hanlon]

Thanks for the info Wade. I cut and pasted it over on the OldGMCtrucks web site as the 55-59 GMC smaller than 3 ton V8 trucks are equipped with Pontiac engines.