Over the years, we thought we knew a few tricks about how to set up a typical Eaton 12-bolt, clutch-style limited slip, better known as a Posi. But we’ve learned a slick trick from Tom at Tom’s Differentials that makes that clutch Posi not only work much better but also last longer. It turns out this differential rebuild procedure can also be applied to other new or used clutch-style limited slips, such as the Ford 8.8 and the Ford 9-inch Traction-Lok. While this story is focused on the GM 12-bolt, the differential rebuild procedure is the same for the 8.2 and 8.5 10-bolts, although some part numbers will change. A typical, new Eaton clutch-style limited slip relies on a thin side-gear shim and a set of coil springs to preload the clutches, which produces an adequate amount of torque to drive both rear wheels. This works for a while until the clutches and gears wear. This added clearance reduces the torque capacity and usually results in tire spin. Tom’s Differentials prefers to custom-build these units, beginning with seating the new gears and clutches by rotating the Posi 50 turns in each direction, then adding a thicker shim package. This requires repeating the process of shimming and spinning until eventually the entire differential is set to the proper clutch preload. This blueprinting process also eliminates the need for preload springs, which, according to Tom’s, reduces the load on the clutches and increases their durability. This differential rebuild procedure establishes a preload you can feel as you turn the case on the shaft. Tom says you’ll know when you have it right. We took a few new parts and our slipping, used Posi over to Moore Automotive, where Tim Moore went through the differential rebuild procedure. If you like moving pictures, you can watch a how-to video that Tom’s has produced (search “Tom’s Differential Posi Rebuild” on YouTube). It’s important to mention that this is a procedure Tom developed. He could have kept it to himself but chose to share it with other car crafters. Armed with this knowledge, you can now do the labor yourself and save a little money you can then use to buy more parts from Tom. It’s a concept that deserves support. Parts List Description PN Source Price 30-spline side gears C12-1017 Tom’s Differentials $185.00 33-spline side gears C12-1017-33 Tom’s Differentials 220.00 Hard cross-shaft TCS-C12 Tom’s Differentials 30.00 Thrust washer C12-TW Tom’s Differentials 4.75 (ea.) Tom’s shim kit C12-PSS Tom’s Differentials 20.00 18-disc plate kit C12-PP 18S Tom’s Differentials 110.50 Bare 30-spline case Call Tom’s Differentials 315.00 Tuned Eaton 30-spline Call Tom’s Differentials 885.00 Stock Eaton 30-spline Call Tom’s Differentials Call Select-fit C-clip Call Tom’s Differentials 6.00 (ea.) Lucas 85W-140 lube N/A Tom’s Differentials 7.00 / qt. GM Posi lube G-PA Tom’s Differentials 11.35 The Other Guys This technique will also work for other Eaton clutch-type limited slips, as used in the GM 8.2 and 8.5 10-bolts as well as the Ford 8.8 Eaton unit. In fact, the Eaton 8.8 uses the same clutches as the GM 12-bolt, and Tom’s offers hardened 8620 steel cross-shafts for these units. The Ford 9-inch Traction-Lok is also rebuildable with new clutches, gears, hardened cross-shafts, and a similar shim pack from Tom’s, so you have plenty of opportunity to upgrade a used limited slip. End Source Tom’s Differentials; 208/265-8111; TomsDifferentials.com If your clutches are in decent shape, Tom recommends reusing them, but only if they are the solid-face style (left). He prefers these to either the segmented style (right) or the latest carbon-fiber version. If your clutches are in decent shape, Tom recommends reusing them, but only if they are th There are three different side-gear tooth counts: 16, 17, and 18. Tom says the 17-tooth (left) is the strongest. The 17-tooth side gears require specific pinion gears. Tom sells new 17-tooth 30- and 33-spline side gears. The spline count must match your axles. A 33-spline side gear can be used in a 30-spline by boring the case. There are three different side-gear tooth counts: 16, 17, and 18. Tom says the 17-tooth (l Start each side-gear assembly with a plate with the two outside locating tabs followed by an internal-spline clutch disc. Lube each plate with gear lube; Tom prefers Lucas 85W-140. The final count for each side gear will be five tabbed plates and four splined discs for a total of nine. This is the count of a typical 18 clutch-plate system. Start each side-gear assembly with a plate with the two outside locating tabs followed by Tom notes there is a nub or ribbed portion of each clutch plate, and these should be facing down when installed on the side gear. This allows the discs to nest more accurately. Tom notes there is a nub or ribbed portion of each clutch plate, and these should be facin The key to setting up a clutch-style limited slip is the selection of shims. Tom sells a kit with a broad selection of shims from 0.010 to 0.045 inch in 0.005-inch increments. With new clutches, Tom suggests starting with a 0.040-inch shim for each side gear. With our used clutches, we combined a 0.020-inch shim with a 0.030-inch shim to create a 0.050-inch package. Always place the shim between the case and the side-gear clutch plate. The key to setting up a clutch-style limited slip is the selection of shims. Tom sells a k Make sure the shim package is on the side gear, position the carrier with the ring-gear flange facing down, and insert the side gear and clutch pack up into the ring-gear side of the case so the clutches don’t fall out. Rotate the case 180 degrees and insert the second side-gear pack. Now slide the Posi over the axle stub in the vise, holding the upper side-gear pack in place. You must align the two pinion gears before rotating them into the case to ensure the cross-shaft will slide through both the case and the pinion gears. The final step is to back one pinion out at a time to insert the thrust washer between the pinion and the case. Tom prefers the steel washers to the OE brass versions. We used one of Tom’s hardened cross-shafts. Make sure the shim package is on the side gear, position the carrier with the ring-gear fl With new gears and new clutches, the Posi case is difficult—but not impossible—to turn using a spanner wrench on the ring-gear boltholes. Tom recommends rotating the case 50 turns in each direction to wear in the new components. If the case turns easily, a thicker shim package is required. Our case turned easily, so we increased the thickness by 0.005 inch on each side to 0.055-inch shims and eventually to a 0.060-inch total shim package. A second Posi rebuilt this way (with used plates and gears) required 0.070-inch shims on both sides. With new gears and new clutches, the Posi case is difficult—but not impossible—to turn usi Tim Moore didn’t want to waste time turning the Posi by hand, so he built a fixture for his Bridgeport mill that held the case and turned the gears at around 50 rpm. This quickly beds in the gears and allows you to shim the clutches to the proper preload. Tim Moore didn’t want to waste time turning the Posi by hand, so he built a fixture for hi If you find a used Posi at the swap meet or in the junkyard, be aware that GM used different ring-gear flange positions to compensate for the changing diameter of the pinion gear. This photo shows the difference between a three-series on the left and a four-series Posi on the right. The four-series carrier moves the ring gear closer to the smaller diameter pinion gear. Also note the severe scratches on our four-series case. When Tom blueprints a Posi, he polishes the case to remove these potential stress risers. If you find a used Posi at the swap meet or in the junkyard, be aware that GM used differe By Jeff Smith Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!