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continued axle leaking

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My first test & tune day with my new square fitment did not go so well. My new setup for the Boss consists of 18x11" wheels, NT-01 305s, Boss T springs, adjustable panhard and rear chassis brace, 22 mm RSB. My day ended 20 minutes into the first session due to a failed left rear axle seal. The right axle seal was also beginning to spit gear oil. Coincidence or something in the setup? Only 19,000 miles, and 25 track days. I'm thinking there is too much softness in the rear, maybe leading to axle wrap and stressing the seals to the point of failure. Solution may be stiffer rear lower control arms, reinstall the Boss RSB, but go with a larger FSB. Any thoughts?
Axle seal leaking should have nothing to do with how soft or hard your suspension is. I think you instaled new seals and bearings, correct? When you drive the seals in make sure the spring does not fall off inner seal lip and use a small amount of grease on the seal lip. Also, make sure your axle is venting. If it doesn't, oil heats up creating pressure and will blow the oil past the seals. Is there lots of play in the c-clips? Also, as mentioned earlier, do not overfilled with oil.1/2" below the fill hole max. I forget to ask, how was the sealing surface on the axle shaft. No grooves or ridges?


TMO Addict?
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10-20 Years
Always pack spring side of a seal with chassis grease to keep the spring in place during installation.
What do the rear brakes look like? I've seen rear brakes hot enough to cook the seals.
Axles look fine, very smooth, detected no warping. C clips do not have excessive play. The brakes do get hot! The Boss vents the rear end on the top of the diff cover, versus the axle tube. While the vent was not clogged, it was hard to blow air thru. Cleaned with brake clean and it's much better. Maybe that was the culprit. Looking at the JLT rear end expansion tube as opposed to the breather type catch can. See Going back to the track next week and want to make sure I address the problem and don't have a repeat seal failure.


Spending my pension on car parts and track fees.
Exp. Type
Exp. Level
10-20 Years
Poor venting would be my guess. I went with the Shelby system. They had a sale, I bought the can.
Also pay attention to the oil fill level. I always fill with the forward plug out. When oil drips from the forward plug, you are full. If you fill to the rear filler hole, you are over filling.

Have you removed the backing plates for the rear brakes? They hold in heat.
Just go to an open vent setup, without a valve. Put a hose on the left axle tube and route it up along the gas fill hose into the tirewell, do a couple of loops and zip tie it there. I have a fuel filter at the end to prevent dirt going in the axle housing. I think the turkey baster has some sort of a valve on top that could potentially fail and not vent.
This sounds like a good next step for re-routing my leaking vent tube.
On my first test and tune in this car, the axle seal failed. Seals/bearings are replaced, I don’t want that again. So I order up the Ford replacement for the rattle cap vent and the 6-8” tube with plastic vent and straight barbed end fitting is delivered (DR3Z-4022-A). After a short 15 mile test drive at highway speeds the vent is pushing out fluid. Two things to do are 1) make sure fluid is 2.5 quarts (dealer did seals/fluid so likely there isn’t an ounce too much fluid in there) and 2) route the breather end way up high as FD Sako suggested.

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