Need help identifying aftermarket diff housing and diff gearing

[powder]

I know it is an LSD because it is marked with a painted S, but when I rotate input shaft/measure output stubshafts I am having a hard time measuring as accurately as I would like. Previous owner removed the tag on the cover bolts which is normally used in identifying it. Previous owner also installed an unknown (and super shitty) aftermarket diff cover.

Can anyone identify the diff cover? Can anyone identify the diff ratio from the markings?


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[pbonsalb]

I dont think the ratio is stamped on the case. It is usually on a metal tag. If that is gone, you will have to measure turns of outputs to turns of input. If it is on the car, drive it and back into the ratio through mph and rpm.

[powder]

3600 RPM at 81ish mph on 245mm Hankook Ventus V12 Evo K110 yields a diff ratio of 3.38. Excellent - PO didn't screw that up.

No ideas on diff cover?

[CMG]

I think it's more accurate to measure turns. Just go three full ones then if it's before quarter mark it's 3.15, at the quarter mark is 3.23 and after quarter is 3.38. You should know if you have a 3.38. Dunno bought diff cover but doubt it's crappy. Finned covers are usually fairly pricey.

[alwaysinboost]

diff cover looks to be a knock-off rogue engineering piece.

[pbonsalb]

3600 RPM at 81ish mph on 245mm Hankook Ventus V12 Evo K110 yields a diff ratio of 3.38. Excellent - PO didn't screw that up.

No ideas on diff cover?

3.38 is a stock E36 diff size so it is popular, but people have built diffs with 3.46 and it is possible someone who went through the trouble to fit the fancy finned cover built the diff. I would use the spreadsheet at www.diffsonline.com under the technical tab to dial it in if you cannot do so by marking and rotating. Could be you have something fancy there. You can also check the breakaway torque on the diff to determine the number of clutches. www.bimmerdiffs.com may have some info on this.

No idea on the cover. Looks cool. RE used to sell covers that looked like that. Why is this one shitty?

[RRSperry]

3.38 is 3780 rpm at 80mph. (245-40-17) So you probably have a 3.23.

The Rogue cover is a knock off of the Alpina diff cover... Either way is a nice piece and not cheap...

[powder]

3.38 is 3780 rpm at 80mph. (245-40-17) So you probably have a 3.23.

Using the TireRack Revs/Mile listings, the 245-40-17 is 806 revs/mile (when new). I'm not sure you used the deformed radius (rolling radius) when you did your calculations.
3780(rev/min)*60(min/hour)/3.38(input revs/output rev)/806(output revs/mile) yields 83.25mph.
3780(rev/min)*60(min/hour)/3.23(input revs/output rev)/806(output revs/mile) yields 87.12mph.
I think I have a 3.38 is my best guess using this method, since my speedo reads almost 81mph at 3600RPM using my math. I will try measuring input/output revolutions again later today.

The cover does not seal because it is not flat (does not appear to have been machined flat). Also, several fittings were threaded straight into the aluminum, without timeserts. The PO then overtorqued them and spun/stripped those fittings. I talked with Rogue, whom it appears to have been a knockoff of, but they said they don't coat their diff covers, so this must be someone else's. The cover leaks from almost every hole, and around most of the gasket.

Can I use a stock gasket when replacing? I assume the contact surface is the same as stock because the diff case is the same so the cover must fit up to stock...

[hellrot328m]

Just pop the cover off and count the damn teeth. That's so much easier and accurate than counting the turns. Get a paint pen and mark the first tooth you count and stop counting once you've got back around. After your certain about the number of teeth on the pinion and ring gear, divide them. Divide the number of ring gear teeth by the number of pinion teeth. Super easy and if you stand the diff up on the input shaft you can do it without losing or draining the oil. And RTV is cheap enough. Some people prefer the proper siff housing gasket though.

Also if you don't want that crappy cover. I'll gladly trade you an OEM one. I'll even sand blast it and make it all pretty and shiny before shipping it your way.

[RRSperry]

It comes from having a 3.38 for 10 years, along with gps. My speedo, obc, and gps agree withing 1 mph. Fwiw...

80 mph is 3750 rpm.

so either your speedo is off, (very possible, see what the obc says when you reset it), or you don't have a 3.38.

use the gear calculator on the diffsonline site.

[diffsonline]

The cover is a TC Design cover. Tony made them briefly and then discovered it wasn't worth the expense.

[powder]

The diff is a 3.23 - 42 teeth on the ring, 13 on the pinion.

Any idea why the diff cover has a funky elbow off where the vent should be? It doesn't have a check valve or anything, it just vents to atmosphere. But now that the PO drilled it that large, and then stripped the fitting, the next size up tap doesn't fit - there isn't enough meat on the boss. I can clean up the threading on the current size, but I think it is going to leak again, and I think it is going to strip again. Ideas?

[MauiM3Mania]

The elbow may have been plumbed to a puke can.

[RRSperry]

Have it welded then tap it for the correct size, or use JB Weld.

[ScotcH]

The elbow may have been plumbed to a puke can.

Very likely. Common on track cars.

[MauiM3Mania]

The cover does not seal because it is not flat (does not appear to have been machined flat).

How did you address the lack of 'flatness'?

[hellrot328m]

Maybe they had a diff cooler on it? I've seen people do that before with an electric pump and small finned cooler in the trunk.

[wanganstyle]

the fabled TC design rear diff cover; If the item does not seal then it was not precision ground in the manufacturing process.

Honestly for cars racing or doing HPDE events under 30 minute sessions an aftermarket cover is not really needed.

If your car is so violent that you break the OEM 188mm e36 M3 stock cover then the European boomerang item from s50b32 euro + a e32 188mm diff cover plate is the simple stronger solution.

One key point that nobody likes to talk about regarding the Rogue finned differential cover; It is HEAVY; much much heavier than the stock BMW part.

NASA GTS is pegged by power to weight................



If one looks at the Ring gear there will be a CODE stamped into it; IIRC off top of head

h39 is 3,38 ratio
H30 is 3,46 ratio
h22 is 3,91 ratio - most common for our norcal track cars

Using the TireRack Revs/Mile listings, the 245-40-17 is 806 revs/mile (when new). I'm not sure you used the deformed radius (rolling radius) when you did your calculations.
3780(rev/min)*60(min/hour)/3.38(input revs/output rev)/806(output revs/mile) yields 83.25mph.
3780(rev/min)*60(min/hour)/3.23(input revs/output rev)/806(output revs/mile) yields 87.12mph.
I think I have a 3.38 is my best guess using this method, since my speedo reads almost 81mph at 3600RPM using my math. I will try measuring input/output revolutions again later today.

The cover does not seal because it is not flat (does not appear to have been machined flat). Also, several fittings were threaded straight into the aluminum, without timeserts. The PO then overtorqued them and spun/stripped those fittings. I talked with Rogue, whom it appears to have been a knockoff of, but they said they don't coat their diff covers, so this must be someone else's. The cover leaks from almost every hole, and around most of the gasket.

Can I use a stock gasket when replacing? I assume the contact surface is the same as stock because the diff case is the same so the cover must fit up to stock...

[powder]

How did you address the lack of 'flatness'?

The cover might actually be flat (will see when installed correctly)- it appears whoever built/rebuilt/"fixed" the diff did an incredibly crappy job with this. There was tape, along maybe 40% of the sealing surface!? Rubbery tape - similar to electrical tape. Some RTV on the outside, but not on the contact surfaces - in some places. I used a plastic scraper to remove most of the junk, cleaned it up with a gasket scraper and brake cleaner, found a stock gasket, and will use RTV in addition, per TCD instructions.

Puke can will be installed later this week - fittings arriving Tuesday. Hopefully this was just a bad day for the PO and everything lines up/seals correctly this time.

the fabled TC design rear diff cover; If the item does not seal then it was not precision ground in the manufacturing process.

Honestly for cars racing or doing HPDE events under 30 minute sessions an aftermarket cover is not really needed.

If your car is so violent that you break the OEM 188mm e36 M3 stock cover then the European boomerang item from s50b32 euro + a e32 188mm diff cover plate is the simple stronger solution.

One key point that nobody likes to talk about regarding the Rogue finned differential cover; It is HEAVY; much much heavier than the stock BMW part.

NASA GTS is pegged by power to weight................



If one looks at the Ring gear there will be a CODE stamped into it; IIRC off top of head

h39 is 3,38 ratio
H30 is 3,46 ratio
h22 is 3,91 ratio - most common for our norcal track cars

The mating surface does not appear to be ground - it was milled. End mill machining pattern still visible. Car is not being used at events over 30 minutes - it is being used at AutoX. But the parts were on the car when I got it. The car is not so violent as to break diffs - I had the rear subframe out to replace bushings, camber arms, and determine the cause of diff fluid leak and it appears I have found several sources. I'm not running NASA GTS so...thank you for the information but power to weight isn't the absolute top of my priority list right now.

And the diff appears to be a 3.23 based on counting the gears. I didn't look at the stamping on the ring gear but I can check that out next time I have access to it.

[NeilM]

The cover might actually be flat [...] The mating surface does not appear to be ground - it was milled. End mill machining pattern still visible.

The original paper gasket is none too wonderful. As long as you solvent clean the mating surfaces you can use Permatex Form-a-Gasket for the diff cover, or even better, Curil T by Elring. Of course if the cover really wasn't machined flat that won't be good enough.

Neil
96 M3 - Rogue Engineering Diff Cover

[Moron95M3]

OP,
Interested in Trading cover for OEM +Cash?

We're building a budget restricted race car and could be interested...

[powder]

The original paper gasket is none too wonderful. As long as you solvent clean the mating surfaces you can use Permatex Form-a-Gasket for the diff cover, or even better, Curil T by Elring. Of course if the cover really wasn't machined flat that won't be good enough.

Neil
96 M3 - Rogue Engineering Diff Cover

I went with the Permatex hi temp (red).