Do 8.8" rear diff's. under late model F-150's have metal or plastic (carbon fiber) diff. covers? I have never stuck a magnet to the one on our F-150. My 1994 Ranger (with 8.8") has a Carbon Fiber cover.

If 8.8: rear diff's under F-150's have the plastic cover: maybe this is why so many tend to leak?

 

my 2001 f-150 has a metal cover ( painted obviously). Probably aluminum.

 

What Mf150 said. It is aluminum. I just changed my fluid this weekend and took the cover off. Mine is a 01 F150 with the 8.8 3.55LS rear-end.

As far as the leaking goes, mine has not leaked however, I can see how they might. Mine as well as all the 8.8's that I know of come from the factory with only silicone no gasket. Before doing my rear-end I called around, including Ford and no one makes one for it. When I took the cover off there was hardly any black silicone on it from the factory. When I put mine back on I used the high temp (orange) silicone RTV and put a 1/8 - 1/4" bead all the way around. It made a great seal, you could see just a little bit around the complete cover where as with the factory silicone I only seen it in some areas.

 

Obviously you did not call the right places.I could have gotten my gasket at (5) different auto parts stores,but settled for Discount Auto Parts.They are merged with Advance auto parts,which also had it.Autozone ,Napa,and a mom and pop auto store also had them.Cost,6.00.I'll be changing mine this week with Redline 80w140 synthetic instead of the dreaded FORD 18.00 aquart bulls***!!!

 

You may be right. I guess back east no one has them. I called Autozone, VIP auto and 3 different Ford garages and no one had one or knew of one for the 8.8. They all told me the same thing that the 8.8 don't need one and it didn't come from the factory with one so no big deal.

A gasket is good, but I can say the RTV works by itself just as well. I paid $7.99 a qt for the Royal Purple 75W-140 gear oil and $7.99 a qt for the Royal Purple Max ATF trans fluid. I bought it online from northernautoparts.com

 

When re-installing the plastic or carbon fiber rear cover on the Ranger with the 8.8" diff., something that was very distinctive was that the cover (where it mounts to the diff.) had a small relief trough all the way around the diameter. This trough was about as wide as a pencil lead.

I used the permatex "high-temp", red sealer. My many years experience in differentials & transmissions in large trucks & off-
road equipment has made me shy away from gaskets. Gaskets tend to leak (after tightened down).

 

I use red RTV in conjunction with a gasket...... never had a leak!

 

Huh, didn't know the Rangers had plastic diff covers. I know my 8.8 F150 is metal. When I changed fluid I used gaskert and RTV to be sure I didn't get a leak and have to loose almost 3 quarts of $14 Amsoil 75w140.