Clube BMW Portugal

So I've been doing tons of research lately on building diffs for the E36. I got hooked after helping to build a Ford 10.5 inch diff, so I started my search. There is a lot of obscurity out there because no true published DIY exists for BMW diffs, other than the few TIS sources for the stealership mechs.

The E36 came with three different differentials. All E36 318i/is/ic came with a "small case" diff. This featured a 168mm ring gear. These diffs can hold up to anywhere between 200 and 300 ft-lbs of torque (specific to ratio). The E36 323i/is/ic, 325i/is/ic, 328i/is/ic, and M3 (U.S.) came with "medium case" diffs. This had a 188mm ring gear and was capable of holding anywhere from 350 ft-lbs of torque on up to 500 ft-lbs (specific to ratio). The E36 M3 (Euro) features a "large case" diff with a 210mm ring gear. These could hold from 500 ft-lbs to 675 ft-lbs of torque (specific to ratio).

Known ratios that will work for the 188mm E36 diffs are:
2.79 (great for FI cars that make lots of torque)
2.93 (stock in 96+ non-M manual E36s, great for FI)
3.07 (rare/premium, great for FI)
3.15 (stock in 95 E36 M3s and E36 325)
3.23 (stock on 96+ E36 M3s, good upgrade for manual non-M E36s)
3.25 (found in some E30s)
3.38 (stock on 96+ E36 M3s, good upgrade for all manual E36s)
3.45 (rare/premium, low tooth contact)
3.46 (considered the *best* all around upgrade for E36 non-M and M cars)
3.64
3.73 (found in some E30s)
3.91 (very common ratio found in automatic non-M E36s, "gas guzzler", lots of shifting)
4.10
4.27

***Low Tooth Contact is an alternative that BMW used in the 1980s. Basically it has taller, fewer, thinner (ID to OD), and thicker teeth. It has less parasitic drag than a regular gear set. The only known set available is in 3.45. These go for a premium.***

Now to avoid confusion and provide clarity too, a lot of people seem to get the definitions of high and low ratio gears mixed up. A low ratio gear means that the pinion has fewer teeth than the ring gear. A high ratio gear means that the pinion has more teeth than the ring gear. In terms of our cars, all of our gears are considered to be low ratio because the ring gear has more teeth than the pinion. The ratio is always the number of teeth on the ring gear divided by the number of teeth on the pinion. However, even though all gears in our differentials are low ratio, the high and low terminology still exists. Low is used to refer to gear sets in which the ring gear has more teeth, and high is used to refer to gear sets in which the ring gear has fewer teeth. For example, 2.93 is a higher ratio than 3.07 and 3.64 is a lower ratio than 3.46. Higher ratio gears are great for top speed and gas mileage, while lower ratio gears are great for acceleration and torque. Generally naturally aspirated cars want to run lower ratio gears because it increases acceleration. Forced induction cars want to stick with higher ratios because they already have the acceleration, and the higher ratio gears can handle greater amounts of torque because the teeth are thicker on the ring gear.

Some building basics

The big things that need to be considered when building a diff are pinion preload, pinion gear teeth contact on the ring gear, and backlash. Here's a simple definition of each:

Tooth Pattern on Ring Gear and Pinion: You will need gear marking compound to do this. Paint up a few sections of the ring gear with the marking compound. While give resistance to the ring gear with one hand, turn the pinion with the other hand. This will give nice impressions of the pinion teeth onto the ring gear allowing you to see the contact pattern. Be sure to turn the pinion in both the drive and coast directions. Both are important. You want the pattern to be fairly centered on each tooth in the coast and drive directions, and neither to high or low. There should still be marking compound left between the worn area and the top of each tooth. For used gear sets, concentrate on the pattern on the coast side of the gear only. The gear marking compound is safe to leave on the gears and does not have to be cleaned off prior to reassembly.





Backlash: This is the slop between the ring gear teeth contacting the drive and coast sides of the ring gear. It has a small acceptable range, usually in the thousandths of an inch and must be measured using a dial indicator. To measure, the pinion must be held from rotating at the flange. The dial indicator is placed onto the diff, touching one tooth of the ring gear. It must be zeroed. Take one hand and try to rotate ring gear. You will notice slop in the gear. The reading on the dial indicator is your backlash.

Measuring backlash using a dial indicator


Pinion Preload: This is the rolling resistance of the pinion gear alone. It is advisable to set the backlash and contact pattern on the ring gear before continuing to this. To find this you assemble the pinion gear with associated bearings, crush sleeve, and any shims. The carrier (LSD unit or open unit) is removed temporarily. The pinion collar nut is slowly tightened and then the pinion is rotated using a needle type inch pound torque wrench. There is a small window of acceptable values for this. Going too tight on the pinion collar nut will raise the preload. This can lead to destruction of the forward roller bearing on the pinion, creating metal shards that will slowly impregnate themselves into the other bearings, eventually destroying the rest of the differential.

Pinion Preload Measurement


Typical Pinion


Though this video isn't a BMW, it explains the idea of pinion preload:
http://www.youtube.com/watch?v=8APYO2sZyJU

BMWs use the same crush sleeve concept with shims behind the roller bearing races to set the pinion depth. Though the guy in the video used a dial inch pound torque wrench, you can use the old beam style torque wrench just as effectively:



Important to the E36

Here are some important specs for E36 188mm differentials (others similar):

Ring and Pinion Backlash = 0.06mm to 0.14mm (0.0024in to 0.0055in - Metric Mechanic believes ideal backlash is 0.003in to 0.0035in, which falls well within the BMW TIS limits)
Pinion Preload = ~12in-lbs to 23in-lbs according to TIS (Metric Mechanic claims 14in-lbs to 16in-lbs)
Carrier Preload = ~11in-lbs to 23in-lbs according to TIS (Metric Mechanic claims 9in-lbs to 11in-lbs)
Axle Cap Bolts = 10ft-lbs + 40 degrees angle torque (with Loctite 242)
Rear Cover Bolts = 34ft-lbs
Limited Slip Carrier Cover Bolts = 25ft-lbs
Ring Gear Bolts = 110ft-lbs (with Loctite 271)

Here are some very informative links that I have stumbled across:

http://www.metricmechanic.com/pdfs/m...al-booklet.pdf

http://www.r3vlimited.com/board/showthread.php?t=92714

http://www.bmwtechinfo.com/repair/main/941en/index.htm

http://tis.spaghetticoder.org/e36/M3_(S52)_COUPE/

http://www.e36-tech.com/forum/rebuil...tial-4264.html

Part Numbers:
**Asterik denotes a non-BMW part number. It is specific to the part in general, but not brand, e.g. Timken HM88542 or SKF HM88542. Once you have the bearing and race part numbers, the sky is the limit. I refuse to pay $73 per bearing through the stealership for an SKF bearing, the same one I can find for $22 with the generic number.

**HM88510 Pinion Race x1 **
**HM88542 Pinion Bearing x1 **
**LM503310 Carrier Race x2 **
**LM503349A Carrier Bearing x2 **
**HM89410 Pinion Race x1 **
**HM89449 Pinion Bearing x1 **
33107505602 Axle Shaft Seal x2
33111214144 Axle Cap O-Ring x2
33139065734 or 33139065735 Shim Sets for Carrier
33101214099 Pinion Shaft Seal x1
33121744368 Pinion Crush Sleeve x? (Depends how experienced you are. If you've done it before, you can suffice with one. If not, I'd have at least two on hand.)
33129065179 or 33129065180 Shim Sets for Pinion
23211490120 Securing Plate for Pinion Nut x1
33121200258 Pinion Collar Nut x1

Brands for roller bearings and races:
Timken
SKF
FAG
National
BCA
Fafnir
Koyo

Tools Needed:
Shop press
Race/Seal driver kit
Dial indicator with clamp or magnetic base
MAPP gas torch
Small cut-off wheel (air powered)
Bearing puller
Metric sockets up to 30mm
Ratchet
Impact
Assortment of screwdrivers
A few punches and chisels
Hammer
Large brass drift

I'm putting this out here as a feeler so that more people have an understanding of what goes into building a diff, and that IT IS POSSIBLE TO BUILD ONE!