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  1. #1

    Default 1966 Bonneville rear axle pinion seal apparently = hen's teeth

    After tracing a knocking noise to the rear axle assembly of my car this past summer, I looked into getting the rear rebuilt, and quickly realized that this was something not for the faint of heart & that a shop should do it.

    By great coincidence a 1966 Gran Prix posi rear unit came up for sale on eBay not far from me. Plus, I am a cheapskate and know my limitations.

    I have gone through it, cleaned it up, found it tight & smooth & have replaced the bushings. (Ya, I know it looked like seven kinds of hell, but it was sealed & full of fluid - the internals are nice)

    The rear axle seals are ready to be done, but I have hit a snag.

    The pinion seal was leaking like a sieve. I intended to replace it anyway, but it has to be done before I can change out the assembly.

    Unfortunately, I cannot find a pinion seal anywhere. I have tried RockAuto, NAPA, and Kantner as well as some generic site on eBay (Ames does not even sell one). The eBay ones weren't even close in either inside or outside diameter. The other three suppliers sold the same seal - the inside diameter was fine (slipped snugly over the yoke neck) but the outside diameter is small by nearly 1/8" and rattles around on the housing.

    Kantner insists I don't have the correct axle (!)

    Now, I have checked both units physically as well as confirmed with the 1966 Pontiac Chassis Service manual that the axle housings are the same regardless of whether the drive is Hotchkiss or Saf-T-Trac (in other words, you can remove your standard rear and stick a posi unit in the same housing), and that the Gran Prix is the same as the Bonneville (and Catalina, and Executive, etc etc).

    For the record the measurements taken with my trusty vernier calipers are as follows:

    I.D. : 1.768"
    O.D.: 2.90"

    I would owe a debt of gratitude to anyone who can point me in the right direction. I can't move ahead without this damned seal.

  2. #2


    I went & sanded a ton of grime off of the original seal and found a part number: 9778370, made by National Seal (I believe they're part of Federal-Mogul now). A brief search online revealed that these seals are for a *1965* axle, and not a 1966. SO it appears I have a 1965 posi axle unit. I've ordered a couple, and one NOS as well off of eBay, so we'll see in a few days.

  3. #3


    date code on axle housing will confirm model year. Can't quite read it from your photos.

  4. #4


    I also have a 66 Bonneville that I rebuilt the rear and found that the pinion seal is a 65 number. It must be that they had a lot of 65 rears left over and installed them in the 66 cars.

  5. Default

    I just went through this same ordeal when restoring my 1965 Catalina. It seems that the rear axles had all interchangeable parts except the pinion seal (which was 1965 only). Maybe it was a poor design and they switched to a different part?

    I purchased a N.O.S. GM pinion seal from ebay which turned out to be made by National. Unfortunately that's the only option for now. Paid about $15 plus shipping for mine.

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