Mark Williams’ Brute Strength Rear End – Part 3

12 Bolt - 1

Over the last couple of issues, we’ve taken a detailed look at Mark Williams honking 35-spline street-strip, iron case 12 bolt. If you go back and look over the previous segments, you’ll see this is a “take no prisoners” package. It’s based upon an incredibly beefy custom build housing and backed with M-W’s special Pro Street axles. This time around, we’ll look at the differential. And the truth is, a 35-spline streetable 12 bolt isn’t possible without this piece from the folks at Detroit Locker.

The good news is the Detroit Locker is the most durable and dependable locking differential on the market today. Years ago when cars were first fit with “lockers” they weren’t exactly well behaved, and they gained a reputation for being noisy (and “clunky”) inside the car. Here’s why: The Detroit Locker functions as an automatic locking differential that is designed to lock both wheels of the axle together automatically with power input, when forward or reverse torque is applied. Both driving wheels are provided with 100% of the power. When the Detroit Locker “locks”, it functions like a spool, solidly linking both wheels together. When torque isn’t applied, the differential “unlocks”. The locking and unlocking process are definitely apparent.

Mark Williams Rear End, Tech, Wayne Scraba
: Here’s a look at the custom 35-spline Detroit Locker installed in the heavy duty Mark Williams’ 12-bolt case. It’s hard to miss the huge caps found on this special case.


But much of that has changed. Giant Eaton Corporation purchased Tractech Holdings (the manufacturer of the Detroit Locker) in 2005. Eaton’s engineering staff went through the Locker and upgraded it. These new Detroit Lockers are now sometimes referred to as “soft lockers”. And while they will still let you know when they lock and unlock, let’s just say the action isn’t as violent as it used to be. In fact, Eaton’s Detroit Lockers can prove rather civilized.

Mark Williams Rear End, Tech, Wayne Scraba
This photo, courtesy of Eaton Corporation provides an exploded view of a Detroit Locker. The primary hardware, from left to right includes the side gears, outer springs, driven-clutch assemblies, spider assembly and center cam. The center cam is held in the spider assembly with a snap ring but is free to rotate.


Today, you can call Mark Williams and buy a Detroit Locker for a wide range of applications, including the very special large bore 35-spline job for a 12 bolt.

Fair enough, but how does it work? Internally, the Detroit Locker assembly consists of the side gears, outer springs, driven-clutch assemblies, spider assembly and center cam. The center cam is held in the spider assembly with a snap ring but is free to rotate.

Mark Williams Rear End, Tech, Wayne Scraba
Mark Williams recommends (and supplies) 85W140 Torco Differential Lubricant for use with the Locker. Additional friction additives/modifiers are not required. By the way, Eaton does not recommend the use of any synthetic lubes with a Locker.


As the street-strip car is under power and travelling forward, both axles (and of course, the back wheels) are locked in position and turn at an identical speed. The vital part of the Locker is the series of teeth found on the spider assembly and the driven clutches on either side of the spider. The driven clutches ride between the side gears, which are splined to fit the axles and turn at the same speed as the axles. This spider assembly within the Locker replaces the spider gears used in a conventional open differential.

As you probably know, a car with a locked differential or a spool doesn’t work too well when it encounters a corner. With a Detroit Locker, as the care enters a turn, the outside wheel must travel a distance that proves to be greater than the inside wheel. This causes the wheels to turn at different speeds. The teeth on the spider and driven-clutch assemblies are cut at a negative angle. In this case, the center cam (which is inside the spider gear) locks into position and functions as a ramp, which disengages the right driven-clutch teeth from the spider gear. This process allows the right wheel to rotate faster around the corner. As the driven-clutch rotation equalizes and both wheels now travel at identical speed, the exterior springs force the internal teeth on the spider and driven clutch assemblies to mesh. This means they now “lock”. In turn, this is the reason why a Detroit Locker sounds and feels like it’s locking and unlocking (simply because it is).

Mark Williams Rear End, Tech, Wayne Scraba
If you choose to use a Detroit Locker on the street, keep in mind, even in the newest “soft” configuration, this isn’t a hush quite setup. According to Eaton: “Because Detroit Lockers have “backlash” or “slack” between the drive and driven teeth and you will hear this in everyday use going through corners and when going from drive mode to coast mode. Also with the vehicle on the ground and the transmission in neutral you will have 1/4 to 1/3rd of a turn of lash in the driveshaft, this is completely normal.”


When all is said and done Eaton’s new “soft” Detroit Locker proves to be the toughest, strongest differential setup available anywhere. The custom 35-spline job used in Mark Williams’ brute strength 12-bolt goes under Eaton part number 187S196A. It mandates a large bore rear axle center section and is available for gear ratios of 3.08 to 3.90:1 (we’re quite sure 4.10:1 sets for a three series carrier will also work). FYI, Eaton also manufactures 12-bolt Detroit Lockers for more conventional 30 and 33-spline applications. The 30-spline setups function with a 1.29-inch axle shaft diameter (at the spline) while the 33-spline jobs work with a 1.41-inch axle shaft diameter (at the spline). Finally, the honking 35-spline job M-W uses has an axle diameter of 1.50-inches (at the spline), which is the same as a Dana 60. For comparison’s sake, a stock small Ford 9-inch (28-spline) has a stock axle diameter at the spline of 1.20-inches while a stock big spline 9-inch (31-spline) has a stock axle diameter of 1.30-inches. Now you know why this special 12-bolt setup from M-W is so strong. For a better look, see the accompanying photos.

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  1. Mark Williams’ Brute Strength Rear End – Part 4 - RacingJunk

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