After servicing my differential (and helping 2 other friends do theirs) I was perturbed that they all seeped fluid around the bottom of the cover, especially after a few months of driving.

I'm sure everyone has read the threads where you use whatever color Permatex and let it dry overnight to make the seal, etc. That is a pain, and after letting all 3 trucks sit overnight to dry, they all 3 seeped gear lube.

So, I was telling a mechanic friend (who works at a GM shop) about this and he told me about this stuff they use for ALL sealer-related work. He said that after the cover is back on and tightened, they fill the differential right then and there - they don't even wait for the suggested 1 hour minimum drying time to elapse.

He said to mop up the extra fluid in the bottom of the differential with a rag so it wouldn't drip; clean the cover and the housing really well with a wire brush; clean the cover and housing with Brake Cleaner and then use this stuff to seal it up:





I used about 1/2 the tube - I made a solid bead on the inside edge of the differential; looped around each bolt hole; made a second bead along the bottom half of the differential and then filled each bolt hole as well.

It dried for 2 hours while I went to get some gear lube and friction modifier and ate dinner.

Came back, mixed the god-awful smelling friction modifier with the Mobil 1 lube, filled it up and there is absolutely no seepage, leaks, drips, etc.

While this stuff is expensive ($19 for 2.4 OZ), it is better than any other sealer I've used in the past.

Just wanted to share...

 

I was told that using anything but the Motorcraft synthetic gear lube and sealer can cause issues with the sealing.

 

Oh that is rich, the GM dealer using a Ford product on all their work. I guess we now know what the Chevy drivers are bragging about. The Ford sealant in they're trucks.

Doug

 

Use a gasket and you'll never have those leaks.And,you'll save about $12.00 by not having to buy that over priced sealant!

 

The Permatex black sealant worked just fine for me. drained my diff and cleaned off the diff cover. scraped off every last ounce of the old seal with a razor blade. used brake parts cleaner to ensure all the old lubricant was off the mating surfaces. Put a 1/4 wide bead all the way around the mating surfaces, making sure the bead ran to the inside of all the bolt holes. Positioned the diff cover back up, bolted everything back down and torqued. Filled it up with lube and took off less than an hour later. No leaks and that was over 6K miles ago. No need for high priced ford sealants.

 

Please tell me where you bought that gasket ! Fel-Pro doesn't make one and Ford doesn't offer one either.

 

Funny - myself and my friends that I mentioned bought the same exact black Permatex sealant. We scraped and cleaned it to perfection and installed the cover just like you described. We added the fluid a few hours later and it was leaking by morning... When we pulled the cover, the Permatex was watery and breaking down across the whole bottom of the differential. I went to their web site and they say to let it dry for a minimum of 2 hours, with the preference being to let it dry overnight. I guess you are a lot luckier than the 3 of us were....

Maybe the fluids we are using are different, because 3 of us had the exact same results. I've also read dozens of posts on here about people having real issues getting the Permatex to set up and hold - most stating they ended up waiting overnight to have any success.

 

I just got done redoing the bearings on my 9.75. I used black RTV that came with the master kit. The tube said to let it stand for 10 minutes and then bolt it up. So I let stand then bolt it up and drove it for 16 miles. This morning no leaks???

 

I let the sealant set for about 5-10 minutes before bolting it back together. And used Mobil-1 75-140W. Maybe I was luckier? I dunno.

 

Pep Boys sells them for $7.00.Advance Auto sells them for $6.00.CarQuest sells them for $7.00.And,they are all FelPro gaskets,part number ;
RDS-55341.

 

when i serviced mine i reused the same gasket that came on the truck stock and added redline gear lube, never had a leak out of it since, in fact this thread reminds me its about time to do front and back diffs. again
Bill

 

That part # you reference is for the smaller 8.8" differential - Lightnings have a 9.75" differential, which Fel-Pro DOES NOT make a gasket for.

The Lightning does not come with a gasket - Ford seals them with the sealer I posted about in the beginning of this thread....

 

I resealed my 9.75 a couple of weeks ago with the high temp RTV black.

No leaks so far! Although I used a tap on all of the bolt holes and I believe that the cover bolt holes are 'blind' holes. Meaning that oil should not be able to leak from the threads.

So, my thinking is that if you are putting sealant in/on the cover holes, you MAY BE limiting the bolt from completely seating (compressing a liquid - law of physics). It could be why the covers begin too leak so soon.

Just a thought.

 

Ditto. I had the same thought.... you should never put the sealant IN the holes... I've blown out castings because some putz before me got overzealous with the sealant and glopped some in the holes.

There's no need for a $7 gasket that you can't re-use anyways. The cork ones will weep as they allow the cover to flex too much, and the rubber ones swell when exposed to gear oil. Plain ol' RTV black or blue have always worked well for me. In the 6 diff covers I've changed/replaced/reused, they've always leaked with a gasket, and never leaked without one. The key is getting both surfaces clean, hosing them down with brake cleaner to remove any gear oil residue, and letting it cure for a few hours before refilling. I've never had an issue when using that technique.

-Joe

 

I agree. I never use a gasket. While gaskets usually don't drip, they will seep a little lube over time. The cover can be distorted during tightening with a gasket. Slicone will bottom the cover to the housing with little/no chance of distorting the cover. This is especially true with the aluminum covers used on early 8.8's.

Silicone will never leak if done correctly. IMO it's 100% better than a gasket. I clean both sides with brake clean after scraping. I like to use The Right Stuff silicone. It comes in a cheese wiz type of can so it doesn't go bad. I spread the bead (about 1/16") on the cover around the outside of the bolt holes and install. I never wait for it to harden. I want it to flow some during tightening. I don't put a huge bead, because I don't want it coming loose inside the case. Good aim is important. Most leaks are caused by smearing the bead during install. I've never had one come back for leaking even when I was still working at a shop. My brother did have a leak on his Astro van, because his diff had a low point drain he didn't notice when spreading the silicone. He got it right the second try.