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OBX Differential Inspection and Installation


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#21 FlatBlack

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Posted 21 May 2010 - 09:43 AM

Here are some photos of the 5MT with an OBX in it, but it doesnt show the orientation of the gears:

http://i19.photobuck...rx/IMG_0119.jpg
http://i19.photobuck...rx/IMG_0120.jpg

As you can see, the axle splines are on the diff itself, as the axles do not clip in, but are attached via compression pins.

Will I need to shim the diff, from side to side, axle to axle? Shimming the bearings I mean?

As you can see in the photo, the bearing looks to be pressed on all the way to the housing.

Will I need new bearing for the axle shafts?

New seals?


It would be best to replace the carrier bearings, I'm going to use my old ones though.

For non-Subaru people, the front of the transmission is in the right of the picture. The axles come off the tranny directly after the motor:

Posted Image

Your carrier is setup like the R160/R200 if I'm thinking about this correctly. The ring gear is on the driver's side of the carrier, like the rear diff

Posted Image

So, it *should* be the same, and the gears need to be in the same orientation.

How bad would it be to run with the gears turning the wrong way?


The gears will 'slam' into the retainer/washer stack on acceleration, which is most likely the reason people complain about unmodified OBX LSDs obliterating the stock washer stacks.

From KA-T.org [A forum for turbo 240SXs]:

I have been running an OBX diff for about 18 months or so. Initially it works pretty well but after 6 months it wore out and was basically an open diff. At the time I did not have the cash to upgrade, so I took it apart and found the spring washeres inside were all worn, craked, and half of them were in pieces. Not knowing were to find replacement beveled washers, I simply put regular flat washers inside. I added enough washers to make sure their was going to be enough resistance to lock up the diff. Immidiately the diff worked great again.


The RBryant washers are much stronger, but it's only extra insurance. It's much better when the washer stack only sees pressure on engine braking [Like the Quaife unit].

Edited by flatblack280, 21 May 2010 - 09:54 AM.

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#22 jc8587

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Posted 21 May 2010 - 10:05 AM

How the hell does this work? I mean, I'm sure your right, but think about the following scenario via MSPaint!

With both ring gears on drivers side, the rear moves opposite the front.:
Posted Image


With rear diff ring gear on passenger side, they move the same way. (ALTHOUGH I AM ALMOST 100% POSITIVE THAT THE REAR DIFF HAS THE RING GEAR ON THE PASSENGER SIDE ALSO!!!):
Posted Image

Edited by jc8587, 21 May 2010 - 10:08 AM.


#23 jc8587

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Posted 21 May 2010 - 10:15 AM

I just thought of this too, maybe the center diff rotates the driveshaft oppostie of the lower gear shaft, makeing it ok to have both ring gears on the drivers side?!

#24 jc8587

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Posted 21 May 2010 - 10:19 AM

Posted Image

Edited by jc8587, 21 May 2010 - 10:26 AM.


#25 jc8587

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Posted 21 May 2010 - 06:13 PM

The gears will 'slam' into the retainer/washer stack on acceleration, which is most likely the reason people complain about unmodified OBX LSDs obliterating the stock washer stacks.



Ok, under acceleration you want the diff to lock up, right? So you want the axle gears to be pulled into the diff and touch the washer stack right?

So the axle gears should be "slamming" into the retainer/washer stack, right?

OR, will the gears be pulled in regardless of the orientation?

The reason we want to run the gears as said is because the barrel gears will crash into each other on acceleration if we do not?

#26 FlatBlack

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Posted 21 May 2010 - 09:33 PM

Oh man. Totally missing a few key points. Go back to the first post, and click on the links Trumpet posted in the original post.

You DO NOT want the washer stack getting slammed into under acceleration. The differential motion happens EITHER WAY.

The Subaru front diff is located beneath the initial gearset. It is not attached to the gearset. The front axles have to turn a certain way to propel the car forward, obviously.
05 Subaru Forester XT - STi swapped - DD
96 S14 KA24DET - E85, MS2E, HLSD, GSP Gen 2 arms, coilovers, TT brakes -new autocross toy

#27 jc8587

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Posted 24 May 2010 - 10:24 AM

Oh man. Totally missing a few key points. Go back to the first post, and click on the links Trumpet posted in the original post.

You DO NOT want the washer stack getting slammed into under acceleration. The differential motion happens EITHER WAY.

The Subaru front diff is located beneath the initial gearset. It is not attached to the gearset. The front axles have to turn a certain way to propel the car forward, obviously.


Yes, I was very confused.

I understand what you are saying now, but it still isn't clear to me which way my gears should go in.

Also, setting the backlash correctly has me a little scared to attempt te swap on my own. I don't want to ruin my transmission.

#28 FlatBlack

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Posted 24 May 2010 - 10:41 AM

I understand what you are saying now, but it still isn't clear to me which way my gears should go in.


The front diff's ring gear is facing the same direction [and has the same gear cut] as both the Subie and Datsun rear diffs.

The front and rear diffs in a Subaru are going to have the same forces [directionally] on the carrier under acceleration. This means you need to flip the gears like we do for our RWD Z cars. take a look at all of the pictures again, and it should make sense.

Edited by flatblack280, 24 May 2010 - 10:42 AM.

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#29 jc8587

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Posted 25 May 2010 - 09:32 AM

So when checking backlash, do you need to hold the pinon gear still, and then rock the ring gear back and forth on the pinion while measuring the tolerance?

#30 cygnusx1

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Posted 25 May 2010 - 11:21 AM

Wow this has really gotten out of hand.

#31 FlatBlack

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Posted 25 May 2010 - 11:31 AM

Wow this has really gotten out of hand.


Agreed. I'm sorry I helped it get that way.

If the Mods feel like it, anything after post #18 could be moved into a "Subaru front diff OBX" swap thread, or just taken out.
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#32 jc8587

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Posted 26 May 2010 - 04:14 AM

Woah... Sorry guys. I guess I didn't realize I had killed the thread, but don't you think this could be used as a good resource for athoer people who want to swap in OBX diffs to their cars?

#33 FlatBlack

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Posted 26 May 2010 - 01:17 PM

Woah... Sorry guys. I guess I didn't realize I had killed the thread, but don't you think this could be used as a good resource for athoer people who want to swap in OBX diffs to their cars?


Subaru front differentials not apply :D

Okay, I dropped my spare 3.54 R200 and modified OBX carrier off at a machine shop this morning, with the ring gear bolt "sleeves" and these torque specs [From the 78 FSM, page PD-25]:

Ring gear to drive pinion backlash: 0.13-0.18 (mm) [0.0051-0.0071 inches]

Ring gear bolt [use locktite]: 6.0-7.0 (kg-m) [43-51 ft-lbs]

Carrier bearing cap bolt: 9.0-10.0 (kg-m) [65-72 ft-lbs]
05 Subaru Forester XT - STi swapped - DD
96 S14 KA24DET - E85, MS2E, HLSD, GSP Gen 2 arms, coilovers, TT brakes -new autocross toy

#34 cd1105

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Posted 30 July 2010 - 08:00 PM

So does anyone have finished picture with the obx installed and/or videos of the obx installed(like a burnout)?

#35 FlatBlack

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Posted 02 August 2010 - 12:18 PM

So does anyone have finished picture with the obx installed and/or videos of the obx installed(like a burnout)?


Um. It'll just look like an R200?

I have a picture of my car with a snapped U-Joint when i took it to the track after installing the OBX. Can't find it at the moment though.
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96 S14 KA24DET - E85, MS2E, HLSD, GSP Gen 2 arms, coilovers, TT brakes -new autocross toy

#36 Jonas240z

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Posted 02 August 2010 - 12:26 PM

Just finished installing the OBX unit on my R200, torqued everything to spec and now backlash is .002 only. It turns smootly but i'm concerned about the smaller lash. Is it ok to run it like that or do i have to adjust it? If so how do i know what size spacer to use to adjust it? Thanks
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#37 FlatBlack

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Posted 02 August 2010 - 01:01 PM

Just finished installing the OBX unit on my R200, torqued everything to spec and now backlash is .002 only. It turns smootly but i'm concerned about the smaller lash. Is it ok to run it like that or do i have to adjust it? If so how do i know what size spacer to use to adjust it? Thanks


I had mine adjusted at a shop, unfortunately I don't know what to tell you.

Here is a fantastic write-up by a Dirty Impreza member [who also owns an S30] about the defects/manufacturing defects that are so common in our OBS units.

http://www.dirtyimpr...1318#post231318
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#38 JMortensen

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Posted 02 August 2010 - 02:49 PM

.002 is not enough lash. You'll need to reshim the ring gear to the left to open it up. Take some shims to your local gear shop and see if there is anything close in outer diameter, then buy a shim kit for that model car.

While you're at it, you can shim in some more preload on the carrier bearings. Get the backlash correct and then add some shims equally to each side so that the carrier doesn't just fall into the housing. The tighter the carrier is the better, because that preload on those bearings keeps the ring gear in alignment with the pinion.

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#39 Jonas240z

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Posted 02 August 2010 - 03:47 PM

Thank you for the help. I think my carrier bearing well preloaded since it was a pretty snug fit to get the OBX unit in. I was just wondering if i just take measurements of the stock shims and the go purchase the next step up from that measurement on the left shim. Will the differance between the two move it just that much to the left? Say mine is .255 and the next one up is .260, will that open the backlash .005? Thanks again, you guys are great help! ;)
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1973 240z Black, future project

#40 camarojokester

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Posted 27 August 2010 - 06:05 PM

hey, I know this might be a dumb question, but where might I find these C-clips




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