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1939 differential leak
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Hi, just wondered is it a straightforward job to put a new oil seal on a 1939 Super Eight differential where the driveshaft connects or is there a trick to changing it. It’s a fairly bad leak but the bearing behind it and seal was changed only 5,000 miles ago.


Posted on: 5/23 16:14
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Re: 1939 differential leak
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The job is complicated by the fact that after changing the seal you must re-establish the correct preload on the pinion bearings or risk premature wear of the ring and pinion gears.

Posted on: 5/23 17:21
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Re: 1939 differential leak
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It shouldn't leak after 5 K miles. Sometimes the surface the seal runs on is worn or pitted, which can damage the seal pretty quckly. Loose bearings or an out of balance driveshaft will shorten seal life. Another thing to check is the vent for the axle housing. Most all rear drive axles have some sort of vent to relieve pressure and allow moisture to escape. They can become clogged and allow pressure to build in the axle housing and force oil past even a good seal. Some vents are hard to find. Some are a drilled bolt with a little tin cap that doubles as a hold down bolt for the brake line junction block mounted on the axle housing.

Posted on: 5/23 21:45
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Re: 1939 differential leak
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This is a reoccurring forum subject. I suggest you use the Advanced Search feature and use 'pinion seal' as the criteria and restrict the search to the forums only.

One of those search 'hits' will be a step by step procedure I wrote in response to a question much like yours, see:


As long as the rearend is healthy there is really no intrigue in changing the seal. It does require a good amount of care returning the pinion bearing preload to the same value that you started with, but it has been done many times.

Later shop manuals and service documents will have a lot more content than the '39 versions. You might read a few of those documents.

There was another posting on the same subject where the question was "How big of an impact wrench should I use?" The answer was "Avoid the use of impact equipment when changing the pinion seal."


Posted on: 5/23 22:00
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