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Old 10th June 2018, 08:58 AM   #1
tombance
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Default Diff Sealing Questions

Hi Guys,

I'm currently driving my M3 Evo about once every two weeks. And when I first drive it out of the garage, this is on the floor every time:



Now a little history about the car. When I first got it (about 6 years ago), it was leaking from all 3 diff seals. At the time I didnt know how to fix that myself, so I took it to a well reputed garage near Southampton, who dropped the diff and replaced the seals. About a week later I started getting this small black puddle under the car after longer drives. I took it back to the same place, and they told me that it was now leaking from where the propshaft CV joint meets the diff input flange. They took it all apart, cleaned it up, applied RTV and out it back together (sounds like they didn't seal it properly first time they dropped it, but nevermind).

There was still a small 'spray' leak after that, but no drips so I was happy.

Roll on another 5 years and one of the diff seals had started to leak again, so I dropped the diff myself and replace all 3. I put it all back together with a new propshaft to diff flange seal, and now I am getting this same leak again. I have crawled under and confirmed it is NOT leaking from any of the 3 diff seals. It is dripping from the metal heatshield that goes under the proposal CV joint, which means it can only be coming from that CV joint to diff flange join. Now the obvious answer is that I didn't correctly seal that joint, but I have some questions:

1) Should diff oil be leaking down the input shaft splines and into the diff input flange? I.e. is it normal for that CV joint to be full of diff oil and not just grease? I actually applied some thread sealant on the input shaft splines and nut when reassembling the diff to try to stop this, but it didn't seem to stop it.

2) if not, what could be wrong with my diff that is allowing oil to travel into that front CV joint when it shouldn't?

3) If that IS normal, has anyone got any tips on how to seal that propshaft CV to diff input flange joint? I assume just lots of well applied RTV?
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Old 10th June 2018, 10:17 AM   #2
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The seals have two sealing surfaces. One between the seal outer diameter and the casing and the other being the inner diameter of the seal and the pinion shaft itself.

It's unusual for the oil to be only getting into the CV joint because there are 3 barriers , the seal , pinion nut and the locking washer.

If its definitely diff oil then ( which it probably is) then it's without a doubt coming from the input seal. You may have a loose pinion nut or some abnormality letting oil into the CV joint if it indeed in the CV joint , maybe it just looks like it's in the CV joint and your getting a false symptom.

Only way to know is to strip it down.

The inner diameter of the seal can wear a groove in the pinion surface which allows oil leak. So next time take note of the seal location and fit the seal a little deeper into the casing to get a fresh bit of meat to seal against
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Old 10th June 2018, 10:29 AM   #3
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Hi Schooner,

Surely the inner lip of the input seal rides against the outer edge of the input flange, and therefore if that was leaking there I would be getting a leak on the outside of that (i.e. not into the CV joint)? That is not the case, its all dry there.

I also re-did the diffs seals about 100 miles and I spent ages doing that to make sure it was PERFECT. Input nut was torqued perfectly to line up with where it was take off from. So I'm 100% confident there isn't a problem there. And the same problem occurred after this was done by the professionals 5 years ago.

The only way I can see oil getting into the CV joint is along the input shaft splines and then past the locking washer and nut. Like I said I sealed all of that with loads of thread sealant, but clearly it didn't work.

Main thing I need to know is, is diff oil in that CV joint normal or not? Because it might be normal, and its just that I haven't sealed that input flange well enough. Has anyone else noticed oil in there when removing the propshaft, or is it just me?

If its not normal to have diff oil in there then I'm stumped. I do have quite a lot of driveline 'lash' which I've narrowed down to either the diff or the dual mass flywheel. I'm tempted to just get the diff rebuilt, I'm tired of crawling under the back end of this car.
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Old 10th June 2018, 11:43 AM   #4
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It's not normal.

The seal does not seal against the flange. You can take the flange off and stand the diff on its nose and oil still won't leak out of the seal is intact. The seal works against rotating pinion shaft hence the name "rotating shaft seal" as o said earlier a groove can develop here from the rotation. So refitting new seals will still leak of they are refitted in the exact same location as the groove will allow oil between it and the seal.

Oil in the CV joint means that oil is getting passed the seal and through the splines. It's not normal and something is wrong. If you have alot of lash in the drivetrain then it's best to check this asap as a loose pinion nut will destroy a gear set.
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Old 10th June 2018, 11:50 AM   #5
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Sorry schooner but I respectfully disagree. If you look at this photo (stolen from this forum somewhere):



You'll see that with the input flange removed and the diff standing on its nose there is a big gaping hole for oil to leak from. Admittedly this picture isn't off my exact diff, but mine is the same setup. Maybe your E46 M3 is different, I don't know.
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Old 10th June 2018, 12:14 PM   #6
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Quote:
Originally Posted by tombance View Post
Sorry schooner but I respectfully disagree. If you look at this photo (stolen from this forum somewhere):



You'll see that with the input flange removed and the diff standing on its nose there is a big gaping hole for oil to leak from. Admittedly this picture isn't off my exact diff, but mine is the same setup. Maybe your E46 M3 is different, I don't know.
Yes your right.

I'm referring to my e46 diff. I assumed it would be the same.

Non the less it's not nornal

From looking at that picture. The oil is travelling along the splines and into the CV joint. Which means it's seeping out from under neath the pinion nut.

I would say in this scenario. Your seals are working just fine. However the oil of getting past the pinion nut. Means one of two things.

Nut has backed off a bit or thread sealant is letting oil out.

Last edited by schooner32; 10th June 2018 at 12:20 PM.
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Old 10th June 2018, 05:00 PM   #7
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Hi Schooner, complete agree. But I know 100% The nut has NOT backed off.

So therefore i need more thread sealant. Next weekend Iím going to take the propshaft input flange off and take the nut off. Re-seal the thread on the nut then re-seal the joint between the propshaft CV and diff input flange. I think that should stop it.

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Old 16th June 2018, 11:08 AM   #8
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Hi guys, finally got this tore down.

As expected, the moment I undid the 6 screws holding the propshaft to the diff input flange, a load of oil dripped out. I think the rear CV joint was completely full of diff oil, and it was actually spraying out of the front of the propshaft CV joint, not the flange itself.

No leaks from the seals though, and the 32mm nut was still tight and where I left it.

So, next question is, how should I seal this back up? I have some granville instant gasket, and some teflon thread sealant. My thoughts were that I should put a bead of instant gasket behind the nut before I tighten it down, so that will get squashed out when it is tightened. And also put another bead on the front of the nut before I drive the dust seal back in. Does that sound like it will work?

What about the threads, instant gasket or thread sealant?

Any advice would be much appreciated!!
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Old 16th June 2018, 11:43 AM   #9
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The diff oil is getting in there somehow and it should not be in there. The only way it's getting in is past the nut, either under the nut ( maybe there is a little low spot or something despite the nut being tight) or it's channeling oil through the threads.

I would remove the nut , make sure the surfaces are smooth , and put thread locker on the threads to keep the oil inside
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Old 16th June 2018, 04:03 PM   #10
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Thanks schooner. I ended up putting a bit of RTV under the nut flange before I tightened it down, and it nicely squidged out around the side and formed a good bead. Also put a lot of thread sealant on.

Its all back together now (took me about 7 hours in total, I hate that job). I'll let the RTV cure overnight and give it a test drive tomorrow.
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Old 16th June 2018, 07:59 PM   #11
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Diff on my 42k mile Evo was sweating when I bought it. Changed the leaking input seal, and since the diff was out, changed both rear seals also.

Hasn’t leaked a drop since.
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Old 17th June 2018, 06:28 PM   #12
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Mine is not leaking from the seals - bone dry there.

So question for you if yours is dry now - when you put the input flange and nut back on did you apply any sort of sealant anywhere at all (under the nut? or threads?).

I have read many threads about this and no one has ever mentioned needing to seal anything, which is why I never did it.

I'm just wondering if there is something 'strange' about my diff.
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Old 17th June 2018, 10:47 PM   #13
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No sealant. There is nothing else to seal that the oilseal not already seal.
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Old 18th June 2018, 07:02 AM   #14
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I think my diff is cursed
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Old 18th June 2018, 09:04 AM   #15
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Quote:
Originally Posted by MParallel View Post
No sealant. There is nothing else to seal that the oilseal not already seal.
If you look at the pictures above. You can see the oil seal does not stop oil entering the splines and ultimately upto the nut. It's a weird design when compared to the e46 , where the oil seal totally seals off the differential cavity. It seals against the pinion shaft itself. But in the pictures above there is not seal against the the pinion , only the input flange.
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Old 18th June 2018, 11:21 AM   #16
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Quote:
Originally Posted by schooner32 View Post
If you look at the pictures above. You can see the oil seal does not stop oil entering the splines and ultimately upto the nut. It's a weird design when compared to the e46 , where the oil seal totally seals off the differential cavity. It seals against the pinion shaft itself. But in the pictures above there is not seal against the the pinion , only the input flange.
Yeah well. Not sure how it works. It is indeed weird. I've never seen the E46 diff with the input flange off.

All I can say is my diff leaked from the front seal and after replacing it, it stopped leaking. So how it works, dunno, but it solved my leaking problem.



Even if the seal went to and around the shaft itself, it would still allow oil into the splines. That would require a 'splined' seal, which would then obvious be spinning.
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Old 18th June 2018, 11:25 AM   #17
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Yep its quite confusing. I think if no one else has ever had to seal the nut then there must be something wrong with some of the 'hard parts' of my diff. Hopefully my RTV 'bodge job' will keep it inside there.

I've just been for a long hot test drive and parked it. Still looks dry for now but I think even if its still leaking it will take a while for the oil to fill up the CV joint cavity again.
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Old 18th June 2018, 11:41 AM   #18
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I just had a look through my diff rebuild pics and I remembered wrongly!!

E46 is the same and the inner diameter seal seals against the flange similar to above.

I guess there is a hardware issue for the OP. Hope the sealing compound has done the trick
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Old 28th June 2018, 11:50 AM   #19
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My M3 was in for its APK (MOT) and when I was standing under the car, I noticed some weaping/sweating of oil at the front of the diff.

Ok, I replaced this seal in 2013 after I bought it as it has the same problem. The car had been sitting for months.

Now this is my fun car and only comes out accassionally. Like 1250 miles per year occassionally.

Now since this is discussed here, could it be that when the car is standing still for long times, the oil is running to the front seal, building up pressure?

Putting the front end up on every park is a bit outrageous.
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Old 28th June 2018, 12:53 PM   #20
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Happened to mine (front diff seal weeping) after sitting for ~24 months.

Had it and the output seals replaces with OE at the same time and it's been fine since (~15,000 kms).
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