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Differential gasket

936 Views 9 Replies 7 Participants Last post by  camachinist
I am planning to change my 9" 4.11 posi with locker to
3.25 open. Car came with this gear originally.
I checked my local auto parts store and no one carries the
differential gasket. It's the gasket between the differential
housing and center section. Is this part still available?
Most mentioned to use RTV silicone instead of gasket.
Is this a good idea? Or should I stick with gasket?
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I've seen them for sale in a Summit catalog.

Also, check with your favorite Mustang supplier since so many of us have the 9".
You'll likely get RTV suggestions here as well...

I prefer to stick with gaskets....Sometimes I think the world is stuck together with RTV *G*

Any online parts supplier will have them...I surprised that Autozone, Kragens, or NAPA doesn't....try Summit...

I'm assuming your switching complete center sections and that the new one has the appropriate yoke on it for your app. Otherwise, you'll need a new crush sleeve to re-set the pinion that point, a new seal would hurt either...then there's bearings...hehehe

If you want to share the particulars of the swap, maybe we can help further...

Good luck!
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NAPA, summit, jeg's....etc.
I picked one up at NAPA a couple of weeks back. By the way, turns out I've got both a 4.11 and 3.25, tell me what your experience with the 4.11 has been? That's what I was thinking of running, at least initially.

Napa's number for the 9 inch gasket is P27929, for those who might need the 8 inch gasket, it is P27807. Last time I was at Napa I had to look them up myself- they weren't on the computer, the new guys just hate to have to use a book...
i had same problem - Napa, AutoZone and others do not want to bother with special parts. i have a lilttle store (K&S) near me that has prctically no stock but he called Felpro and got me one next day for my 7 1/4 rear end cover.
The problem is since they don't have a specific make / model they cannot look anything up... stupid, any good auto parts store with a guy working there that actually works on cars (hard to find these days) would be able to locate it for you. The gasket is the same for any 9" rear end application, so potentially you could pick any newer Ford vehicle that came standard with a 9" rear.


I really would like your help on this task I'm doing.
This is my first time to remove the center section.
At the same time, the FMX will be replaced with a C4.
C4 came from a '68 coupe and was working when removed.
Should I do a complete rebuild or just install it in my car?
I also have the driveshaft from the '68 coupe.

I will send you a list of part numbers I have on hand.
Hopefully parts I have are all OK.
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You're doing a number of things here, all of which need to be done correctly for a good long-term outcome.

My first suggestion, if you don't already have one, is to purchase a good service manual. I would imagine VMF'ers have discussed this subject before as to which is best. A Haynes manual came with my D-coder and it has pretty good info but I have heard good things about the Chilton manuals, among others.

Regarding the trans...if it came out of a running car and worked well, I would just perform routine maintenance on the unit (drain converter and trans and change filter and re-fill), along with an inspection of the inside of the pan for debris. You could install a shift kit at this time, which would make inspection of the hard parts easy (valve body comes out)...unless there was something obvious going bad, I'd just leave it alone. The trans in the D-coder looked like new inside at 178K when I put a shift kit in it (OEM unit, never rebuilt) and is doing just fine as it prepares to turn the odometer over once again.

I'm not real familiar with FMX vs C4, as far as X-members, driveshafts, etc. but will bet it'll be an easy swap. The service manual will detail the step by step...just remember, if something doesn't seem right or you get stuck, stop, step back and think about it (post a question here if necessary)...don't rush...
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