Overdrive / transmission rear seal 1936-38

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    The overdrive rear seal on our 1601 has been replaced twice with the National felt seal from Olcar listed in the Parts & Services directory.

    This seal is now leaking at an accelerated rate; yesterday, a quart of NAPA GL-1 was added to the transmission after being topped-off just two weeks ago, few miles driven since and a puddle of oil on the garage floor. (The transmission was down a quart but the separate overdrive unit was still full to its fill-plug level.)

    Is there a better rear seal available? If so, please advise your source and part number – thank you – Stu Blair


    I am showing it to be a National #473455 which is pretty readily available and it is nitrile. Below is a site that has the dimensions. Might want to see how they matchup.




    Has the rear OD shaft factory bushing been replaced? That is the cause of the demise of so many overdrives as the “dogs” (can’t think of the correct term) of the OD gear break, because the separate trans and overdrive shafts get out of alignment due to bushing wear at the rear of the OD. Remove the OD (don’t need to pull the trans itself), replace the bushing with a bearing and machine the case to fit, then use a modern lip seal.

    I’m sure Bob Koch can give you much more complete and accurate info.


    Thank you Bill & George,

    The O/D shaft bushing has not been replaced to my knowledge; this car had 31k miles on the odometer when acquired in 2011 and it was up on blocks for 58 years prior to that.

    We’ll try the Motion Industries nitrile seal first.

    Thanks again!


    Stu, while preparing for the seal replacement, after unbolting the rear u-joint, try wiggling the driveshaft while another person watches the back of the OD for shaft wobble within the OD housing. And again try to wiggle the OD shaft after the driveshaft is completely removed. I don’t expect to find any wiggle/wobble, but if you do that’s VERY serious and requires replacement of the bushing with a bearing.

    You’ve driven the car enough to know how much / how frequently there is any clash when engaging OD, so plant that firmly in your memory now. If the frequency or amount of clash increases, “just do it!”” That approx 3/4-inch thick OD clutch gear with 8 “”dogs”” is the hardest part to find in good condition.”


    It’s rather easy to machine the rear transmission yolk for a modern seal. You can polish the seal lip surface in a lathe. It’s possible for oil to leak past the spline, so cleaning and sealing that is also advised.

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