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Leaking Pinion Seal

Ed Martin

Ed Martin

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My 68 roadrunner is leaking differential oil from the pinion seal. It's the original 8-3/4 differential '742' with a 1-3/4 pinion. I drive it about every three weeks and it only leaks about every other time I drive it, probably about a teaspoon. I'm wondering whether replacing the pinion seal will stop the leak or, since it does not leak every time, maybe the pinion shaft is worn such that it causes a leak only when the worn spot on the pinion stops rotating below the oil level when parked. Could the seal cause wear of the pinion shaft? In any event, how likely would just replacing the seal fix the leak?

Any thoughts are appreciated.

IMG_20250427_145538139.jpg
 
The seal rides on the yoke, not the pinion shaft.

Seals go bad, and you need a new one. These days sealer needs to be applied to the outer diameter of the seal.

I like Dr Diff pinion seals.
 
Remove the nut, pull the yoke, pop the seal, wipe the bore clean, smear aviation gasket sealer or red head on the new seal and install with an appropriate sized socket/pipe/seal installer etc. Clamp the yoke in a vise, take some 120 sand paper... a LONG strip is best and buff it like a pair of shoes until it's smooth and "lip" mark free. Reinstall the yoke and retorque the nut, I believe it is 220 ft-lbs, but I'm getting old..
 
Last edited:
The OEM seal is probably leather and could be dried out from sitting/old age (like some women I know). I like the leather but the newer plastic style is also available.
Mike
 
And before driving the seal in pack the back side with grease... It keeps the garter spring from popping off...
 
I just bought a pinion seal from Oreilley's yesterday for a 742 case, P/N PSO 7216. When I picked it up I was surprised it was a leather seal in a
National Federal Mogul box.
 
R413 said:
The seal rides on the yoke, not the pinion shaft.

Seals go bad, and you need a new one. These days sealer needs to be applied to the outer diameter of the seal.

I like Dr Diff pinion seals.
Click to expand...
Thanks for your reply and clarification. I have not had it apart and could not tell exactly how the seal fits together with everything from my factory manual exploded diagram or the online information I've found so far. So the pinion should not be affected, just put in a new seal?
 
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