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Discussion Starter #1
fighting with this diff fluid leak from what seems like the rear cover. i did the output shaft seals a while back and then re-sealed the rear cover as it was leaking also. well now the rear cover is wet again. hard to tell exactly where its coming from as the air flow under the car blows it all around. the only thing i can think of is the orientation of the oil passages in the flanges that bolt to the case. i marked them before i removed them to replace the seals and put them back exactly how they came out. however there seems to be some confusion as to exactly which way they should sit in the diff. mine are above the oil level. if you were looking at the right side the oil passage would be right around the 10 o'clock position and the left around the 2 o'clock position. according to this diy, http://www.m5board.com/vbulletin/e3...erential-side-axle-seal-replacement-pics.html they are not in the right positions though im pretty sure the diff was never touched till i wrenched on it and i marked the orientation of the flanges and put them back exactly how they were. seems a bit odd as why would there be an oil passage above the fluid level inside the diff? has anyone ran into an issue like this?
 

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Friday night I was helping a friend build a E39 M5 diff and we ran into this very issue. When I removed his side covers, the oil passages were facing down and to the front. As far as I was aware, these should face up and back. All other BMW large case diffs I have rebuilt (excludes E39 M5 diff), had those oil passages facing up/back. Old BMW documents also recommend this orientation (see pic below). I have a E34 M5 diff that we opened up on Friday evening and sure enough the passages were facing up/back. The diff has never been disassembled as far as I know.

That said, we could not find any BMW documentation regarding orientation for the E39 M5 diff. Ultimately, we settled on down/front orientation for his covers based on orientation they were in upon removal and the information on the thread you linked. I'd suggest reaching out to Jim at performance gearing or Dan at Diffsonline for a definitive answer.

 

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Discussion Starter #4
thanks man, im not having a problem with the axle seals leaking, its the rear cover thats leaking again and i know i sealed it up good. mine where definitely up and back but im going to try and set them down and to the front and see if it makes any difference. i also took the dremel and opened up side of the seal where it sits against the oil passage. originally the seal was to the depth of having half of the hole open, i ground down the flange on the seal so the hole is fully open. we'll see if it makes any difference. still dont know if these are oil passages or some kind of vent. if you install them up and back they are above the oil level in the diff so idk.
 

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It looks like that hole connects the interspace between the bearing and the oil seal to the main diff housing. From the internet: “A tapered roller bearing has a natural tendency to pump oil from the small to the large end of the rollers. For maximum oil flow and heat dissipation, the oil inlet should be adjacent to the small end of the rollers."

That makes me think the hole should be at the bottom so the bearing sucks oil from the main housing reservoir into the interspace and pumps it through the bearing back to the main housing. And if the brg does create some suction on the interspace, that would reduce the pressure on the oil seal. Although I would have thought 2/3rds of the brg being above oil level would have vented the interspace pretty well and kept it at main housing pressure.

Based on the above, I can't see any reason why the interspace would pressurize and have to be vented by the hole being located above the oil level.

There's no vent hose on the diff, is there? So how does it vent when it heats up?

You should have no leakage from the cover. It looks like it's a bullet-proof sealing surface unless you have some porosity or a flatness problem.

Not a lot of help, I know.

Diff cross section sketch.JPG
 

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Discussion Starter #6
there is a vent for the diff, it has a small hole in the top of the rear cover. i moved them down and to the front with the left pointing around 7:30 and the right around 4:30. im wondering if it wasn't the cover thats leaking but instead its somehow building pressure and pushing some oil out the vent hole, idk,
 

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A high oil level can cause a lot of foaming and aeration which may come out of the vent hole. Did you check the hole was clear? And if it didn't have the internal baffle installed, it would leak from the normal splash.

1221121811_zps08c51157.jpg
 

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Discussion Starter #8
the fluid level is spot on and i made sure the vent hole was clear and the baffle was there when i had the cover off.
 
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