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Finally making my 3.9 LSD..calling out for tips


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8 replies to this topic

#1 88HatchMonster

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Posted 15 December 2004 - 04:01 PM

Hi all, I finally got my 3.7 LSD Diff, a Snap-On E10 female torx, and I should be getting side seals, o-rings and spring pins tomorrow. Oh yeah, got the lock-tite too.

So I'm getting ready to tear into it tomorrow. I've still gotta take my 3.9 diff off, but that's a peice of cake.

Anyway, I figure I'de see if anyone who's done the conversion had any pointers that might keep my out of trouble, ya know just simple stuff that might be obvious to you but not to someone who has never even opened one of these up before (like me.)

I'm going by the directions in the USRM, which are great BTW. And it's going in a 4" lifted hatch w/ 6 lugs, 28.5" swampers and rear discs.

Thanks, guys!

#2 Vanislru

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Posted 15 December 2004 - 04:17 PM

Keep everything seperate, well marked, and take photos as you go. If you can, do it all in one day preferably starting out early on. I will be doing this swap too, but I have experience from other diff's/trannies, organization and physical comparisons [photo's] are key. you probly already know all of this anyways.

#3 Qman

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Posted 15 December 2004 - 06:22 PM

Just follow the USRM to the letter. I have done many and I wrote the document you will be following. Use a couple of shop rags to set everything on. And make sure that the you keep the 3.90 ring gear with the 3.90 side cover/bearing races. It will keep the gear mesh correct.

#4 singletrack

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Posted 29 December 2004 - 05:31 PM

Hi all, I finally got my 3.7 LSD Diff, a Snap-On E10 female torx, and I should be getting side seals, o-rings and spring pins tomorrow. Oh yeah, got the lock-tite too.


I'm also about to do this swap. Where'd you get the seals and rings and such, is it a "dealer only" item, are there parts numbers? Till I take it apart, I can't picture exactly what I'll need.

What about clutch disks? My LSD has around 150K, what's the expected life of these things.

Futhermore, how much grinding needs to be done? I have no air tools, but I do have access to them. What's the suggested method?

#5 Qman

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Posted 29 December 2004 - 06:37 PM

Clutches should be fine. Grinding can be done with an electric drill motor and grinding tool. Stones have even been used. Do you have access to the LSD case? It will let you see how much needs to be ground to fit the LSD unit in the 3.90 case. Most importantly, follow the info in the USRM. Trust me, it will answer your questions.

#6 singletrack

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Posted 29 December 2004 - 09:36 PM

Grinding can be done with an electric drill motor and grinding tool. Stones have even been used. Do you have access to the LSD case? It will let you see how much needs to be ground to fit the LSD unit in the 3.90 case.

Sounds like there's not to remove then. I've got a 3.7 LSD here, I haven't taken it apart yet though. I've got 2 3.9s to work with, so I can have a running car and take my time.

Most importantly, follow the info in the USRM. Trust me, it will answer your questions.

I wouldn't even be trying it without your article, thanks. What about the seals, are these what I'm looking for?

#7 88HatchMonster

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Posted 29 December 2004 - 10:34 PM

I ground mine down with a dremel but that took too long so I got a pack of grinding stones for my air die grinder and that method was a little quicker. I'm sure a drill would have worked fine too. It's just to get the LSD into the case so it doesn't have to be perfect.

I got the seals, o-rings, roll pins for the axles, and a gasket for the rear cover from my subaru dealer. I was gonna get aftermarket stuff but I wasn't sure I would be gettting the right thing so I just went to the dealer. The gasket was a special order and I'm expecting that any day now so I can finish the job.

Do you have the special torx socket for the stub axle bolts? I've heard you can use a 5/16" socket or something like that but I didn't want to risk it so I ordered the stupid expensive snap-on one cuz I couldn't find it anywhere else.

Stock up on brake clean spray and use it liberally to clean parts and clear grindings from the case.

3 out of 4 of my stub axles were seized to my cvs so be prepared to deal with that if you get rust where you live. I had to pop my wheel halfshafts out with axles still attatched and build a contraption to press them off, it was a nightmare and penetrating oil sure didn't help! But that's upstate NY for ya... Oh yeah I also broke off four of the side retainer bolts taking it apart, so don't be like me, heat 'em up if they don't come easy.

My LSD has 150K on it too, but I hear they are tough...

Props again to Qman too, I would have had no idea what to do without your article either. So Thanks! I will really be thanking you when I finally try it out in my car though :)

#8 MilesFox

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Posted 29 December 2004 - 10:36 PM

yo what is the part number for the snapon and who carries them?

sorry to jack...i was helping my buddy tear down an lsd, but his 3/8 dr socket was to no avail, the bolts were rusty anywat, we got them off by torching them with acetylene!

anyway, we torched the bearings so how do you get the bearings off?

#9 Qman

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Posted 29 December 2004 - 11:04 PM

What about the seals, are these what I'm looking for?


Hard to say by the cut away pic they use.

yo what is the part number for the snapon and who carries them?


I don't have the part number handy but what you are looking for is an E10 torx socket. The "E" denotes the female torx. Make sure to get the 1/4" deep socket. Snap-on or Mac are the best bets to get the right one.

Do you have the special torx socket for the stub axle bolts? I've heard you can use a 5/16" socket or something like that but I didn't want to risk it so I ordered the stupid expensive snap-on one cuz I couldn't find it anywhere else.


Using a 5/16" socket has worked but usually results in a broken socket or rounding the edges of the bolt. Using an impact driver is usually the best way to get them off. The kind you hit with a hammer not the air kind.




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