GM 12-Bolt Rear End Rebuild - Detroit Truetrac Limited Slip Differential

A fundamental part of any muscle car is a nice, strong rearend fitted with a limited slip or Posi unit. What good is it to make gobs of horsepower if you can only do one wheel burnouts or if the diff grenades the first time you put the spurs to your new engine? Plus, the best way to transfer that power to the ground is through two tires, not one. Lately the trend has been to ditch the factory rears for a 9-inch, but not everyone wants Ford-style parts on their Chevy. So for all the purists out there, we are going to rebuild a 12-bolt with stronger than stock parts and add a new limited slip differential.

The rear end in question is out of a '66 Bel Air that now has a stroker small-block and a 4L60E overdrive transmission. Not only will we add strength and traction, we'll also change from a 3.08 gear to a 3.73 ratio, which is better suited for acceleration and an overdrive. With the 4L60E's 0.70 overdrive gear ratio, a 275/50/17 tire (27.83 height) and a 3.73 ring and pinion ratio, the engine will be spinning right about 2,000 rpm at 65 mph, which should be perfect for decent gas mileage. We picked up everything needed for the upgrade/rebuild from Moser Engineering and took it all to Steve Post at Diff Works (Corona, California) to have it installed and set up correctly. Once the rear end is finished we'll be able to lay down two fat black marks with confidence.

Here's the stuff we got from Moser to beef up our 12-bolt housing. We will go into more detail later, but here is a quick breakdown of the parts: Detroit Truetrac limited slip differential...
Here's the stuff we got from Moser to beef up our 12-bolt housing. We will go into more de
...Moser 30-spline C-clip replacement axles, 3.73 ring and pinion, aluminum cover, upgraded yoke, and a setup kit.
...Moser 30-spline C-clip replacement axles, 3.73 ring and pinion, aluminum cover, upgrade
A lot of the upgraded strength is going to come from the axles. The Moser 30-spline C-clip replacement axles have a couple of upgrades that make them stronger than stock. First, they are constructed from stronger materials and second, the shafts are non-tapered so there is not a machined-down weak point.
A lot of the upgraded strength is going to come from the axles. The Moser 30-spline C-clip

Another strengthening procedure that Steve at Diff Works does is to radial weld the tubes to the center section. Since the factory only uses three plug welds, the added welding makes for a stronger housing.
Another strengthening procedure that Steve at Diff Works does is to radial weld the tubes
Since we are doing a complete rebuild, all new bearing races will be installed. Steve uses a hammer and drift to knock out the old races.
Since we are doing a complete rebuild, all new bearing races will be installed. Steve uses
Steve passes on this tip to install the new race: Use the old race to drive in the new one. All the races will bottom out in their respective holes so drive them in until the sound of the hammer blow changes pitch.
Steve passes on this tip to install the new race: Use the old race to drive in the new one

For the outer axle bearings, a bearing installer tool was used. Just like the races, these will bottom out in the axle tube so Steve made sure to drive them all the way in.
For the outer axle bearings, a bearing installer tool was used. Just like the races, these
Steve adds a bead of silicone sealant to the outside of the seal before installing it. This will prevent any diff oil from creeping out from around the seal.
Steve adds a bead of silicone sealant to the outside of the seal before installing it. Thi
The new 3.73 ring gear is bolted up to the Truetrac and torqued down to 65 lb-ft then the new bearings were pressed on with a hydraulic press. The car was originally equipped with a 3.08 ratio, but since the car now has an overdrive transmission the gear ratio was lowered to help us get better use of the new gear ratios in the transmission. The Detroit Truetrac is a helical gear-type limited slip differential, so there are no wearable parts that will need replacing like a traditional Posi. Steve said, "The Truetrac is a good all-around unit that is best suited to street/autocross use because it's a little more forgiving than a Posi."
The new 3.73 ring gear is bolted up to the Truetrac and torqued down to 65 lb-ft then the

A 0.030-inch shim was slid over the pinion before the new bearing was pressed on. The pinion shim will dictate the wear pattern or engagement between the ring and pinion, but we won't know if this is the right shim until the rest of the center section is installed and we check the wear pattern-more on this later.
A 0.030-inch shim was slid over the pinion before the new bearing was pressed on. The pini
After the pinion was slid into the housing, the new Moser yoke was greased up to prevent tearing the seal and installed with the provided nut and lock washer. This new billet yolk will now use the stronger 1350 U-joint instead of the smaller and weaker 1310 U-joint the factory uses.
After the pinion was slid into the housing, the new Moser yoke was greased up to prevent t
The newly assembled differential slides into the center section and some shims were placed on each side of the differential to center it.
The newly assembled differential slides into the center section and some shims were placed