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Rear Differential Overflow Valve


Luke Skywalker
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What does that valve do on the differential, and why is the entire upper portion of my differential case coated with grease?

Something doesn't look right.

Related question: I bought a new gasket for the differential cover. It's awfully thin. Any suggestions on how to seal that guy when I get to it?


Luke Skywalker
Probably just the breather doing it's job. Hot oil expansion and a bit of splash over the years you'll get a buildup. If, at any time, it became blocked with dirt the back pressure may have unplugged itself by 'puking' a bit. Pretty normal, how long since you've checked that area?


Luke Skywalker
Are you sure it's grease? The entire top of the diff on my TR4 was thickly coated with cosmolene or Waxoyl. Old and dirty, it could pass for dried up and thickened grease. It was up to 3/4" thick in some of the indentations of the diff housing. I'm sure it came from the factory that way in 1962. I found the same stuff packed all around the parking brake assembly. And, this was the first time some of those parts were fully removed from the car giving access to things to really clean them up, so I know the stuff was "original". I think they sprayed it on liberally to protect the cars from salt air during overseas shipment.

The breather is just that... a small tube that allows air to breathe in and out of the diff. Without it, you'd see leaks at the seals pretty quickly, after the first couple times things heat up. The breather protrudes a little in hopes of keeping any water or dirt from entering the diff. I've replaced it with a barbed fitting and will run a hose to a catch tank with a proper breather on it.

The cover gasket is just paper. It's easy enough to make one if you want something heavier. But it doesn't need to be very thick if the cover is in good condition (see below). I just used a coating of Hylomar on each side, not a drop of oil to be seen.

Check that the flange of the diff cover is nice and flat, and that it hasn't been overtightened and belled up at the bolt holes. If you find anything, straighten it as best possible before reinstalling. Other than that, it's pretty straightforward to install and seal up.


Jedi Warrior
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As noted in the other posts, the diff will blow a little oil out if it gets hot and/or if it has been over-filled (which is usually difficult to do). Or the coating indeed could be old waxoyl or the like. But just to add, check to see that the 'pin' is still in that breather. Essentially the breather is just a hole with a split pin in it. If the pin has come out it could blow a lot more oil out.

On your related question, yes the gasket is very thin paper. But the two surfaces are machined pretty flat. I used a generous coating of hylomar on both faces of the cover, installed the gasket on it and that seems to do the trick. When I took mine apart, it had traces of old hylomar in the bolts holes, etc. so I think that is what was originally used. Just make sure to get the two surfaces real clean before refitting.
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