GM 10 Bolt Rebuild
Speedway Motors employees enjoy not only helping our customers, but also turning some wrenches in their downtime as well. Take Zach R. for instance. Zach got enlisted by a friend to help rebuild the 7.5" factory 10 Bolt rear in the G-Body Monte that he is working on, so naturally it became a teaching moment. Follow along as Zach works over the old 10 bolt and brings it back to life.
If you’re building a street car and want to improve performance, we all know the drill. Upgrade your cam, intake, headers, stall converter, and the list goes on from there. One intimidating part for a lot of guys is the rear end. Sure, you know what’s in there and what it does but are you willing to tear into it and rebuild the differential? Recently a friend enlisted me to help rebuild his 7.5” GM 10 bolt. He is resurrecting a G-Body Chevy Monte Carlo and decided to reuse the original 3.73 gears and a factory GM posi unit.
We started by getting a rebuilt kit from Motive Gear . This ring and pinion install kit comes with new pinion bearings, carrier bearings, races, crush sleeve, pinion nut, pinion seal, a gasket and even has the gear marking grease included. Speedway also offers the Economy Ring & Pinion Setup Tool we needed. As an added reference, Speedway Motors also offers a great book from CarTech, titled "High-Performance Differentials, Axles & Drivelines" that helps with step by step breakdowns of the process.
Before disassembly, we marked the bearing caps and kept the side bearing shims on the correct side. This will ensure that your backlash is correct without having to swap the shims. Next we disassembled everything, starting with the cross pin bolt, cross pin and C clips. After that we removed the old bearings and cleaned all the parts up in the solvent tank. Once you give everything a thorough inspection you can begin the re-assembly process. (Note: The 7.5” GM rear used different pinion bearings in 1979-80 vs. the 81-up axles, so you want to make sure that the old bearings match up to your new pinion bearings).
Many times when you are rebuilding an existing rear end you want to save the original pinion bearing shim and re use it under the new bearing. Some guys even go to the trouble of making a set-up bearing so you don’t have to press on and remove the pinion bearing to try different shims. We pressed the new bearings onto the carrier and the front pinion bearing and went ahead and put the assembly together into the housing. An important step I like to take is to assemble the pinion and differential case without the crush sleeve or the new pinion nut. This allows you to put the pinion gear in, set the pinion bearing pre-load, set the backlash and then run a contact pattern without the chance of compressing the crush sleeve too much.
We were lucky and had a desirable contact pattern on both the drive and coast sides of the gear teeth. However, when you have new gears, a new posi unit, and other variables like mixing housings with differentials you might have to try multiple shims to get a good pattern.
Now that we know our pinion depth shim is correct we can do the final assembly. One thing often overlooked is the RTV sealant that needs to be applied to the splines (otherwise rear end grease can leak past the splines), and locktite on the new pinion nut. I also like to put some oil on the seal surface of the yoke to avoid tearing the new pinion seal.
When compressing the crush sleeve, many guys use an impact wrench and end up crushing it too far, ruining the sleeve. The correct way to do this is to use the yoke lock (part of the setup tool kit we bought) and use a breaker bar with a cheater pipe. I like to slowly tighten the new pinion nut until I take up all of the front to back free play. From there I make very minute changes, checking the pre load with the inch lb torque wrench several times. This is very important, because if you go too far the crush sleeve will be ruined.
Now that the hard part is done you can re install the carrier with the shims in place, torque the bearing caps and run one more contact pattern to make sure everything is good to go. If it checks out you can put your axles back in, C clips on, cross pin and bolt and you are ready to put the cover on and fill it with rear end lube!