'08 Rear Diff Cover - Where Did I Go Wrong?
 

Ssssoooo...

5 1/2 months ago I installed an '08 diff cover on my Sterling 10.50 when I changed my "never needs servicing" rear diff fluid. I followed Guzzle's write-up (Welcome to guzzle's Rear Differential Maintenance Web Page) to change the fluid (I used Mobil 1 75W140 LS synthetic) and Permatex RTV gasket material (red) that is pressure resistant and temperature resistant to 650F. I cleaned everything really well, let the RTV skin up and then let it sit for a few hours before filling up the pumpkin.

Fast forward 5 months (several camping trips, some weekend off-roading and a whole lot of commuting) and 9000 miles later and I notice a small spot on the driver's side along where the cover meets the diff. Over the last week it has gotten noticeably larger. I have been doing city driving for the last month before the spot showed up. I cleaned it up last night and checked the level (still up to the fill hole) so it is only weeping right now.

Did I miss something? Did I use the wrong gasket material? Could the aluminum machining surface just not be matching up very well to the diff housing? I'm going to pull the cover this weekend and put it back on with a liberal amount of gasket material (hopefully I can reclaim some of the fluid, but I'm going to buy 4 more quarts just in case).

Any other tips or tricks that I can use this weekend to make sure I get the best seal possible?

Sounds like you did everything right. You did circle each bolt hole with rtv?

When you have the cover off check the leaking area with a straight edge, both the cover and diff housing.

I think you just had a bit of bad luck.

sounds like you did it right. surface prep is key to a good seal. just make double sure its clean and you should be good to go.

Yep, circled each bolt hole too. I was painstakingly anal about surface prep so that something like this didn't happen. I'll give the ruler trick a try to check the mating surfaces on that side. I have a metal ruler somewhere in my garage, I'll have to dig it out.

Thanks for the info. Hopefully this next try works out better. I have other mods to save up for instead of dumping $80 in the rear end twice a year! It goes without saying that my wife is T-H-R-I-L-L-E-D about this development (this past summer I did the transfer case, tranny flush(and installed a FTVB), front and rear diffs all at once with Mobil 1 for about $300 in fluid alone so that's still fresh in her mind).

FWIW, the local Wally World here has their house brand 75w-140 Synth gear oil for only about $10 a quart. Its the same stuff as the name brands for a heck of a lot cheaper. I assume that the bolts were torqued down also. Just plain bad luck.

I have done a lot of the same fluid changes as you. My wife is also real thrilled any time some minor thing comes up. Geesh! Good luck Brian. I would use most of the old fluid also but that is just me.

Did you try just re-torqueing the bolts again? I would give it a tightening all around before taking it apart again. Now that the silicone is dry, who knows?

Where did you buy the cover? I was thinking of getting one and doing the same thing.

Good luck.

I am going to try to re-use as much as possible, but I'm making sure I have a contingent plan to guarantee completion. This is my daily driver and needs to get me to work on Monday. I don't live close to work so it's hard to carpool without heavily bribing somebody.:)

crawl under with your wrench I would put money on the bolts having loosened up just a tad since you did it.
very possibly the only thing you did wrong was to not retorque them, the rtv can "squish" just slightly especially if you had it very thick at all (I am talking 1/16" of an inch or less.)

This is the one I got:

Ford 12 Bolt 10.25 & 12 Bolt 10.5 OEM Aluminum Differential Cover

I'll try the re-torque. Does anybody know the torque for the steel hex head bolts that go with the new cover (I misplaced my instructions that came with the cover)?

Thanks for the link Brian.

I found this in the 2008 WSM.

If the re-torque doesn't work or if the sealing surface isn't perfect, Fel-Pro does make a differential gasket for the 10.5" rear. Autozone carries it.

Also, there is no such thing as "never needs servicing" differential fluid. If you tow at all you should change it every 75,000 miles....even with the synthetic in there.

Sounds like you were careful & probably just had a run of bad luck. I too always clean my sealing surfaces with lacquer thinner to remove any oil film before using silicone.

Only thing I can add is that I used the black stuff. Doubt that affected things here though. I would try a snug down.

Thanks for the info. Reps sent.

I had no idea. I'll be checking that out today. Reps sent.

I bought a can just for this job. Still have plenty left and will use it again this time. This stuff just melts the most stubborn contaminants away.

OBTW: I checked the bolts last night and a couple of them on the left side moved ever so slightly. So far this morning no new fluid on the cover so it seems to be holding. Since I have the stuff now I am going to do a re-seal this weekend just for my peace of mind. I'd rather spend the time and money now then have it start leaking again when I'm in the middle of nowhere camping.

Thanks for all the responses and help. This why FTE is the best! Have a fun and safe Thanksgiving.