Greetings: I frequent the xt6 site alot however there are many subs which share the same rear diffs. I have seen the two posts on the USMB which cover this, here: http://www.ultimatesubaru.org/forum/showthread.php?t=66430 and here: http://www.ultimatesubaru.org/forum/showthread.php?t=49919.

 

Got a minute?

 

I have disassembled two diffs, the 3.7 LSD from an 89 RX and a 3.9 open diff from my 89 xt6. (actually the one off my parts xt6). There appears to be no problem doing the swap, both ring gears have the same bolt size and patterns and there appears to be no metal which has to be removed in the 3.9 case to fit the LSD innards. However all four end caps (or the aluminum caps with the outer races in them have) two gasket type shims on each of them, I have not measured them yet but they appear identical. The two links above both suggest keeping the 3.9 caps with the 3.9 case. No problem.

 

Here's my question: Since both diffs have about 150K miles on them, do these outer races get worn a little, maybe suggesting removing a shim? There is a very complicated formula in the fsm which uses numbers stamped on the ring gear, diff innards, and measurements from the bearings to determine the number of shims. Too fussy? Is the fsm overkill? How much endplay is OK when everything is assembled minus the rear cover? Really seems to me these diffs are overbuilt so maybe any bearing wear is not an issue?

 

Thanks for your thoughts.

It would be worth re-shimming the diff if either had made funny noises or something but in this case I think its an if it ain't broke don't fix it answer.

the few times I've done it. I've left them as they were.....with zero problems.

the few times i've done it. I've left them as they were.....with zero problems.

 

+1

+2

I've had no problems with the swap in my XT6.

The shims are there to allow you to adjust the pinion to ring gear backlash and contact. If you must, you can put some prussian blue on the ring and check the contact pattern after you put the sidecaps on. If the contact patch is good, don't worry about it.

I've done this a few times (boy that's an understatement) and I was surprized that you didn't mention anything about grinding the diff housing. There isn't much grinding, but it is needed to clear some bolts on the LDS unit.

Thanks, everyone. Makes it a little easier. (:-)

How did the swap go? I've done this many times, only for SVX, using a non-lsd 3.70, 4.111 and 4.444. Swap in the SVX lsd [fyi-different than XT lsd] I have had good success using all the donor diff parts, including end caps and shims. To double check, two things I've learned. First, before disassembly of diff, rock the input hub, and note the travel, sound and feel. If the clearances are good in the converted unit, the feel will be the same. I've used gear marking compound to confirm, but if it feels right, and you use the complete 3.90 with the 3.70 lsd only, you should be okay.

 

That said, I need a 86 or newer 3.70 donor rear diff, doesn't have to be lsd, maybe one of the 'left-overs' from a 3.90 conversion. Can anyone help, I'm in Southern California.

Of course, attach the 3.90 ring gear to the 3.70 lsd center. On SVX, the donor housing nearly always must be ground/relieved to install SVX lsd, I believe on the LHD driver side of the diff.

Actually, I have three nice post-86 3.90 rear diffs. I've inspected the gears and clearances, these are nice units with the larger ring gear bolts.

 

If anyone needs a 3.70 to a 3.90 conversion, I can provide that service.

Need4speed: I received your PM however your box is full. The 3.7 diff is being re-assembled as an open diff and being put back into the donor car. I was thinking about selling the whole car for 500 or so, but I may part it out. The car needs a lot of TLC, but if I decide to part it out, are you saying you want to make an offer on the 3.7 diff (its not LSD anymore)?