Differentials - Introduction

Because they are so long lived and require so little maintenance, BMW differentials are taken for granted by most of us. We at Metric Mechanic do offer rebuilding services on all BMW "short-neck" differentials (approximately '69 and later). Although bearing and spider gear failures account for most of the rear ends we rebuild, we get many requests for changing the final drive gearing (ring and pinion) and increasing the lock-up and/or adding limited slip. Rebuilding includes all bearings, gaskets and seals. Differentials are assembled to low side (tight) factory tolerances for longevity and come with a two year limited warranty.

Gear Ratios
With 4 cylinder BMW's, gear ratio choices are somewhat limited. The 3.45 gear ratio was found in the European 320/6 and the 323i. This ratio has somewhat limited availability but makes a nice final drive ratio to use with one of our larger 4 cylinder engines. The 3.64 and 3.90 rear ends were commonly found on 2002's and 320i's. Late 1600's used a short neck 4.10. Limited slip was really only available on "S" pack 320i's from '78 to '79 (3.64 limited slip) and from '80 to '83 (3.90 limited slip).

Early 6 cylinder differentials were side loaders, meaning that the innards of the differential loaded through a side cover. The bulk of these rear ends from early Bavaria's and Coupes to 530i's and 528i's, were non limited slip and used 3.45 and 3.64 ratios. The Euro 635csi used a 3.07 rear end and commonly these were limited slips.

Later 6 cylinder differentials were rear loading with an aluminum finned rear cover. The '5 & 7 series models - 82 and later; 3 series (with "Baby" Six engines) and 6 series - 83 and later; have a vast number of differential range and pinion gearing options. Limited slip is widely available in this differential style in all "S" pack models.

Ratios
Every model of a given tire diameter, engine size, and transmission configuration will have an 'ideal' ring and pinion ratio for 'best' performance. Depending on whether you want better off-the-line, top-end or overall performance - if you have to rebuild the differential, then it is also the opportune time to consider ratio changes. The following is a break down on choosing the correct MM engine and transmission combination for a given gear ratio.

Limited Slip Options - Increased "Locking"
If you are one of the fortunate owners of a limited-slip, we now offer a special service for your differential that will greatly enhance your BMW's drivability.

Early differentials of this type were set at a modest 40% which seemed adequate for minimal performance needs like ice or snow covered roads. Oddly enough, around '80 BMW reduced the percentage to 25%, which is so minimal (25 ft. lbs. and 45 ft. lbs. breaking torque for 4 and 6 cylinder models, respectively) it negates the sole purpose of the limited-slip option. Since engine displacement and horsepower are constantly increasing it would make more sense to increase the percentage. This is exactly what we can do.
When you send your limited-slip to us we can increase the amount of breaking torque (the force it takes to spin one rear wheel independent of the other) to match your individual driving needs. As you drive more aggressively or squeeze horsepower from the engine, the more essential it is to keep both rear tires from losing traction. The percentage options are as follows:

Values may deviate as much as 5% depending upon condition of friction discs and spring cups

Metric Mechanic's 2002 "Maximum Stick" Differential
Metric Mechanic's Maximum Stick Differential is a limited slip unit contained in an aluminum housing that mounts externally to the differential between the right carrier bearing housing and axle flange.

This exterior location offers several advantages.

Percentage of Lockup
20%, 30%, 40% and 50%. The following pictures show; the assembly sequence of Metric Mechanic's 2002 "Maximum Stick Differential".

Coping with inside rear wheel lift
BMW's in general have a nasty habit of lifting their inside rear wheel under hard cornering. This is one situation that a Quaife (torque sensing) limited slip can't deal with, but a more conventional limited slip has no real problem. For cars with more tail end weight such as a Porsche 911 or a formula car, a Quaife would be an ideal limited slip. Another drawback to the Quaife is the cost. By the time this limited slip is installed into your differential, you can wrap up close to two grand.

The 320i Stock Limited Slip
The factory 320i Stock limited slip comes with a 25% lock-up and can be fitted to a 2002. To do this, you'll need to switch the inside CV Joints to 320i. In 1979, 320i's used 10 mm CV bolts (earlier 320i's used 8 mm). These limited slips are available out of salvage yards for about $400 to $600 and a used CV axle usually runs about $50 to $100. Since most of these limited slips have logged quite a few miles, many times the effective lock up has been reduced. For $250, Metric Mechanic will alter the slip ratio. To do this modification, the rear end and carrier needs to come apart.

To conclude, in looking at your limited slip options, we think you'll see that Metric Mechanic's limited slip looks very attractive.

Installation
Installation is included in the price of the Metric Mechanic "Maximum Stick" (limited slip) differential unit. We can take your good used rear end, machine the right side carrier bearing housing, and install our "Maximum Stick" unit. People who want their rear end rebuilt can also convert to a "Maximum Stick” (limited slip) differential.