changing diff output shaft seal

[fe1rx]

I have a 3.46:1 diff with an M-Factory helical LSD, originally installed in 2015. In 2017 (20,000 km later) it suffered an internal bolt failure which was repaired under warranty. The replacement only has about 3000 km on it now but has been seeping oil from the RH output shaft seal, so I decided to replace it. Here i the process I followed:



The oil seepage was slow but enough to foul up the rear subframe directly behind the diff. The above picture is after a thorough cleaning and a short drive.



First step is to remove the bolts securing the half-shaft to the diff. Lucky for me the leak was on the RH side so I did not have to remove the muffler to get access. Hang the shaft to avoid stressing the outer CV joint.



A pry bar and a spacer will pop the shaft out of the diff. Use care not to damage the dust shield.



The 3.46:1 diff uses a 90 mm seal. p/n 33107505605 includes both the seal and the shaft retaining clip. The normal MT diff uses a different seal.



A standard seal puller will pop the seal out easily. I had a good look at the seal before pulling it out and couldn't see any damage to the seal.



Pulling the seal reveals the output shaft bearing. There is no need to disturb this.



I used a seal driver set like this to install the new seal.



The sealing surfaces on the output shaft looked good but there was a bit of suface corrosion on the shaft.



I polished it up with crocus cloth to provide a perfect sealing surface.



I decided to change the fluid as a precaution, despite the low mileage. I used a siphon tube to easily drain the diff.



If you get the siphon tube geometry just right, it will completely drain the fluid. I made mine from steel fuel line.



The drained fluid looked a bit murky. The internet is full of discussion about diff fluid turning dark. I attribute it to changes in the additive package under stress. Street driven cars may delay these changes but they seem to happen rapidly to track driven cars, regardless of the fluid being used. I am using Castrol Syntec 75W90 GL-5 at the moment but have seen it with Redline and others also.



Torquing up the half-shaft bolts to 52 lb.ft. (per Bentley) I applied torque seal to the heads. This doesn't lock the bolts but provides a visual indication they haven't moved when doing a pre-track "nut and bolt" inspection.

[Stewbie]

Thanks for the DIY. One question. Bentley says to replace the "bolts and washers". Are the bolts of the aluminum "torque to yield" variety?

[fe1rx]

Originally Posted by Stewbie

Thanks for the DIY. One question. Bentley says to replace the "bolts and washers". Are the bolts of the aluminum "torque to yield" variety?

No, they are steel fasteners.

Many of the replace recommendations made with respect to steel fasteners are based on normal 4-season service, where the fasteners are likely to be changed only once, if at all during a vehicle's life. Accordingly, they are likely to be corroded by the time they are changed. If that is your vehicle's lot in life, I would follow the recommendation. For my track car, where things tend to be dissassembled with some regularity, and don't get corroded, I apply discretion based on the condition of the fasteners. Generally, if the fastener comes out without corrosion on the threads or head, and the head isn't chewed up, I don't replace it. I generally reuse self-locking nuts a couple of times, but that is based on observing if the locking feature is still working.

Some BMW fasteners are "microencapsulated" in BMW-speak, meaning they have a paste threadlocker pre-applied. BMW may ask you to replace these just to get new threadlocker on them. In these cases, reusing the fasteners, if in good condition, is reasonable, provided you apply some Loctite on them before reinstalling them.

Torque-to-yield applications will always specify an initial torque, and then an additional turn of a specified number of degrees. Not all such instances are torque-to-yield though. In a true torque-to-yield application, the fastener is permanently stretched on installation, and it shouldn't be reused. If I don't believe* the fastener is truly torque-to-yield, I may reuse it "a few" times.

*that determination requires engineering judgement that isn't readily condensed into a rule of thumb, so you are on your own there.

Perhaps it should go without saying, but I torque every fastener that has a torque spec to that spec. Absent that, none of tha above applies ...

In the specific instance of the half-shaft bolts, these are steel microencapsulated bolts. I have had my half-shafts out several times and reused the bolts several times with Loctite, but in due course the heads were starting to look chewed up so I replaced them.

If the fastener is actually aluminum, always replace it.

[Stewbie]

Thanks for the info!

I guess the micro-encapsulation explains why the bolts are so expensive. $4.42 per bolt on BMW Canada's web-site!

I've got one leaking diff output seal. I figure I might as well replace the other one while I'm in there. So that's 12 bolts for a total of $53.04, which seemed kind of crazy for something that isn't gold plated.

[fe1rx]

If you are anywhere near YYZ, try these guys for parts:

Sean Lowry
Exoparts Inc.
Genuine European Auto Parts Distributors -Since 1994-

3190 Ridgeway Dr. Units #27/28/29
Misssissauga, ON
L5L 5S8

Off: 905-820-2506
Fax: 905-820-1523

My only connection to them is as a satisfied customer. They sell genuine BMW parts but where appropriate they will also offer aftermarket options.

[Stewbie]

Thanks for the tip. I'm a lot closer to YVR than I am to YYZ, so Exoparts won't work for me.