GM 8.5″ 10 Bolt Rear Axle Tech
We do a lot of work with GM 10 bolt rear axles, and specifically the 8.5″ ring gear version found in 1977 – 1996 GM full size cars, station wagons, and trucks. Many people want to change their gear ratio, which can be a great way to improve performance. Changing gears is an involved job and there are several other upgrades to consider while you’re in there.
Note that other GM axles, including the 12 bolt and the 7-5/8″ 10 bolt, tend to use different parts, but the overall construction and design are very similar. If you need help with parts for any other axles please contact us.
Changing your drive axle gear ratio can make a radical difference in how your car accelerates. Compared to the cost of adding more horsepower, changing gear ratio can be a relatively economical way to improve the fun and responsiveness of your vehicle. Here we answer the most common questions we get about the process. And note that if you’re modifying an all wheel drive or 4 wheel drive vehicle, you’ll need to change both front and rear gear ratios in order to keep them matched.
Because of the effort and cost involved to change drive axle gears we like to inform customers of all the options and considerations “while you’re in there” – specifically, certain upgrades like a new and improved differential require very little incremental labor – or zero incremental labor – when done in conjunction with gears. And certain maintenance items are best addressed at the same time – if they need addressing at all.
Rear axles explained here … focused on the GM 8.5″ 10 bolt as found in … tech and parts selection …
Ring & pinion …
ABS reluctor … 3 channel ABS on B-Body (not D-Body); ’91 – ‘0=96
Rear gear install kit – only change is that the bearings (two identical ones) that get pressed onto the carrier have a larger inner diameter to match the Truetrac. Regular install kit is $102. Both install kits use Timken bearings, same brand as OEM.
Total with shipping and after discount is $389 ($385 + $4 to change to Truetrac install kit)
As far as the other items I mentioned please see below – and I threw in a few more for the complete picture. The first two are definitely things to consider with higher mileage.
Inspect axle shafts upon removal – they may be worn, the area that rides on the outer axle bearings can become pitted, particularly the passenger side. we use Moser high strength axle shafts with ARP studs, they are $300 + shipping
Definitely think about replacing your outer axle bearings and seals while in there – $85. It is extra labor but worth it and since you will already have the rear partially disassembled it’s an efficient time to do it. Parts:
Bearing – need 2 – $63.30 for both – https://threepedals.com/products/rear-axle/outer-axle-bearing-for-8-5-10-bolt-rear-sedan/
Seal – need 2 – $21.78 for both – https://threepedals.com/products/rear-axle/outer-axle-seal-for-8-5-10-bolt-sedan/
GM updated the pinion yoke and seal design, adding an extra sealing lip to minimize drips. There is a new yoke and corresponding seal:
Pinion yoke – $80 – https://threepedals.com/products/rear-axle/upgraded-pinion-yoke-requires-revised-pinion-seal/
Revised seal – $17 – https://threepedals.com/products/rear-axle/upgraded-pinion-yoke-seal-requires-revised-yoke/
PCM reprogramming for speedo – with a gear ratio change your speedo will be off. We offer speedo reprogramming for $110.
We can either reprogram your PCM if you mail it in, or we can send you a PCM to exchange, we charge $125 to send out the core and we refund $110 when we get your PCM back in usable condition, for a net cost of $15.
ARP studs – $45 – no extra labor to install these while in there, these replace the carrier cap bolts and add a bit of extra strength – https://threepedals.com/products/rear-axle/carrier-studs-8-5-gm-10-bolt-rear-axle/
TA rear cover – $160 – this adds a bit of extra strength and offers increased fluid capacity which should keep temps down a bit. If you’re making 450+ HP I strongly recommend it but below that this is a “nice to have” not a “must have” – https://threepedals.com/products/rear-axle/high-performance-differential-cover/
Note on rear gear fluid – with the Truetrac no friction modifier is needed as it is gear-driven, not a clutch plate design like OEM.
There are several brands of gears, and some have multiple options. We use Motive Gear G885410 which is their “performance” 4.10 gear set, they do extra lapping on these to ensure quieter operation especially during break-in.
For gear install kit we use Motive Gear R10REMKT which is sized for the Truetrac and includes Timken brand bearings. Some bearing brands are of lesser quality, we like Timken and NSN. Your diff uses Timken from the factory.
The Truetrac is our diff of choice, it is very strong and does not wear out like the factory diff. We use the 30 spline setup, pn 913A481.
There are some additional upgrades we offer. If you’re adding a lot of horsepower and / or racing we recommend these for additional strength.
- Differential cover with preload studs – TA-1807
- Carrier cap studs to replace the bolts – TA-1815
- Pinion yoke with stronger U-joint and improved seal – Suggested if you’re changing driveshaft, or if your current driveshaft pinion end takes a 1350 u-joint –26060881
- Yoke requires a different pinion seal and different driveshaft strap kit
If you’re keeping your existing driveshaft, we have a pinion yoke that uses the stock u-joint size, but is an improved design from GM that adds an additional lip to the pinion seal – 40029412. Requires same upgraded seal as other yoke – pinion seal
For the cover gasket, we love LubeLocker’s reusable gasket – LLR-G850