I own a 1998 F-150 4x4 with a limited slip rear end. The vehicle has about 125,000 miles on it. About two months ago I changed the fluid in the rear axle for the first time. I took the cover off and scraped all the old seal off both surfaces. I used ulta black high temp. RTV sealant to use as the new seal (since they do not sell preformed gaskets for my diff.). I did not allow any time for the silicone to set. I placed the cover back on and immediately tightened the bolts to spec (28 to 38 ft/lbs; I used 30 ft/lbs.). This caused the sealant to ooze out of the edges. I waited 24 hours then added the fully sythetic gear oil and lim. slip additive. I had not noticed the diff. leaking at all until today when I walked back to my truck after driving it for about 30 minutes. The truck had been worked hard over the last week hauling railroad ties over rough terrain and on some rough roads in the mountain last weekend in 4wd. I spoke to the guys at Napa and they all had their own opinions about how to use the silicon sealant. I am trying to figure out if maybe I should only hand tighten the nuts and wait for the seal to set before tightening to spec. If so then how long should I wait? Then I spoke to some guys I trust and know that work for Goodyear and they advised that they do it the same way I do it. The manager then showed me a service book for Chevy's becuase it had a note at the bottom saying that fully synthetic gear oil will eat away at RTV sealants. This note was not on the ford section of the service book. Has anyone heard of sythetic oil eating away at silicone seals? I need some advice on the proper way to use the sealant in regards to how long to wait to tighten the bolts to spec? Any advise is appreciated.

 

Welcome to FTE!

Syn eating away RTV seals is total BS in my book.

I do it exactly the same way you do, and have never, ever had troubles.

You don't mention where it is leaking out, but the way your post is, it sounds like oil is comming out the cover, and not the pinion seal, or the wheel axel seals.

It may be something as simple as you missed a spot on the cover.

What weight oil did you put in? I hope you used a 75-140 and not 75-90....IMO, and personal experince with 8.8's and 9.75's in the newer trucks, don't really like the lighter 75-90 in the rearend for some reason.

I would also check to see that your vent line is not plugged up, that may have caused your new RTV seal to blow out....but usually it causes blow-by in the seals.

Personally, I'd clean up the rear diff with some degreaser, and pull the cover off and try to re-seal it.

If you are carefull you should be able to re-use the oil. You could try to use a suction gun from the filler hole, but that usually takes forever.

 

The leak appeared to be coming from the driver's side of the diff. cover seal. I did not spend much time inspecting it though. Thanks for the advice Rangerman. I used the 75-140 syn. I was wondering where the vent tube was located for sure. I know where the speed senser on top of the assembly is but in relation to that?

 

There should be a black hose that runs from the pumpkin up and into the frame rail, that would be the vent hose.

 

I changed out the rear diff. fluid the same way i did the last time but used a little more RTV sealant. My guess is that I must have missed a spot last time. I spoke to a ford Tech. today who advised that they use RTV silicone on their rear diff. covers. I still have not located a vent tube for my diff. (9.75"). I drove about fifty miles since the fluid change and so far so good. I appreciate the feedback. Sometimes someone agreeing with you is all you need to ease your mind about a worry.