Diff Postmortem with Pics

[redlightpete]

I took apart my 3.73 lsd because it was getting noisy, expecting the pinion bearings to be shot. That was indeed the case with pretty severe pitting on the inside outer race.

The limited slip section seemed to be working fine, but when I took it apart one of the friction discs and the mating carrier piece have some serious gouging/metal transfer.

No wonder the limited slip still seemed to work. Looks like it was on the way to becoming a spool.

Anyway, can I just have the carrier piece ground smooth? I measured .095" of preload in the stack, so it seems like grinding off .005" to .010" wouldn't mess things up too much. Does this surface need to be ground or can it be milled?

I'm also wondering about the ring and pinion. The wear surfaces on both are smooth, and there's no discernible step between the worn portion and the unworn portion of the tooth, but the wear covers almost the entire tooth. In all the 'proper wear pattern' diagrams only the center of the teeth are contacting, like the original surfaces were slightly convex. Is it going to be a problem to get the ring and pinion back in position with new bearings?

Thanks for any advice.

[jlevie]

More than just the center of the ring gear was worn because the pinion position shifted as the pinion and output shaft bearing wore. Unless the tooth shape is badly distorted new bearings all around, shimming the carrier and setting preload/position of the pinion should work.

The clutch plates could be ground or lapped just enough to take off the high spots. That will almost certainly reduce the lock up of the LSD, but it will still work. Best approach would be new clutches.

[Wanganstyle]

I took apart my 3.73 lsd because it was getting noisy, expecting the pinion bearings to be shot. That was indeed the case with pretty severe pitting on the inside outer race.

The limited slip section seemed to be working fine, but when I took it apart one of the friction discs and the mating carrier piece have some serious gouging/metal transfer.

No wonder the limited slip still seemed to work. Looks like it was on the way to becoming a spool.

Anyway, can I just have the carrier piece ground smooth? I measured .095" of preload in the stack, so it seems like grinding off .005" to .010" wouldn't mess things up too much. Does this surface need to be ground or can it be milled?

I'm also wondering about the ring and pinion. The wear surfaces on both are smooth, and there's no discernible step between the worn portion and the unworn portion of the tooth, but the wear covers almost the entire tooth. In all the 'proper wear pattern' diagrams only the center of the teeth are contacting, like the original surfaces were slightly convex. Is it going to be a problem to get the ring and pinion back in position with new bearings?

Thanks for any advice.

Gearing relationship will need to be adjusted when you replace all the bearings/races.

If the gearset has worn into itself from dead bearings/races you may just want to get another set of 3,73 or whatever ratio desired gears (we have raced with 3,91/4,10 happily) ; no amount of adjustment with shims can make up for a worn set of gears.

Your center ramp surfaces show melted clutch disc material smeared on the surface; this is common in e30's and more than 50% of them are destroyed like this. Precision grinding is the best way to make this surface 100% smooth.

Clutch discs are history; order some new ones from your local BMW dealer; they are reasonably priced and genuine BMW.

steel key'ed pieces are also wear items; they need to be replaced; your clutch discs are destroyed and so are the steels.

Measure the amount of stack you grind off the center ramps and adjust the steels to suit the removed material; Porsche sells clutch discs in 2.4mm

[redlightpete]

Thanks for the input. The ring and pinion backlash was still in spec before I took it apart, so hopefully the ring and pinion aren't totally done. The carrier bearings still looked ok, and the pinion wasn't noticeably loose. I guess I'll find out when I check the tooth contact pattern.

On that note, what do you use for checking the contact pattern, dedicated 'tooth marking compound', or can I just use spray paint? Also, do you replace the lame-o pan head screws on the lsd housing with normal hex head cap screws?

Wanganstyle, I though you were exaggerating about these diffs all being 'melted time bombs'. I guess mine is too.

[Wanganstyle]

Thanks for the input. The ring and pinion backlash was still in spec before I took it apart, so hopefully the ring and pinion aren't totally done. The carrier bearings still looked ok, and the pinion wasn't noticeably loose. I guess I'll find out when I check the tooth contact pattern.

On that note, what do you use for checking the contact pattern, dedicated 'tooth marking compound', or can I just use spray paint? Also, do you replace the lame-o pan head screws on the lsd housing with normal hex head cap screws?

Wanganstyle, I though you were exaggerating about these diffs all being 'melted time bombs'. I guess mine is too.

GM gear lash paste. Local Chevy dealer

New top cap screws are a pita to get: if you cant find them locally I have a box here.

Crush washer get from bmw dealer

Races on pinion usually die first; the oil seals can outlast them: 299k on my e34 and the oil seals were still good.

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