When it comes to gears and rears, many concentrate on the high-end, bulletproof differential and axles-and for good reason. When it comes to the transfer of horsepower and torque, a vehicle's rear gear and axles can be the weakest link in the chain.
What if the hot rod is not a racecar or mega-horse street flier, but rather a cruiser running in the mid-400-horse range that will never see the track? Building a rear end capable of withstanding 800 horsepower and 6,000-rpm clutch drops with slicks is certainly no disadvantage, but is it always cost-effective to do so? Throughout the '60s and '70s, GM vehicles came equipped with a myriad of gear ratios, axles, differentials, and so on. Identifying and even modifying these can be a bit flustering, especially if the rear-end came from another car or swap meet. There's no telling what might be inside. Recently Jim Allen and Randy Lyman wrote a book called Differentials, Identification, Restoration & Repair. Before that there was the Chevrolet Chassis Service Manual, and that was pretty much it.
The particular rear end in this story came out from under a 1971 Monte Carlo. According to Jim and Randy's book, the early '70s were kind of a mixed bag when it came to rearend gearing and sizes. The 1971 A-bodies were no different, some had the coveted 12-bolt while others had the 10-bolt and the 10-bolts also varied in size as well, using both 8.5-inch and 8.2-inch diffs. The 8.5 is by far the more desirable of the two because of the larger ring gear and larger 1.625-inch diameter pinion.
As circumstance would have it, the rear end we pulled from the Monte had an 8.2 ring gear inside. Allen and Lyman point out that this rear will work for moderate power outputs, but not serious high-performance work. On the unit we had, one of the axles had been replaced sometime during its life, and it had an 8-inch differential cover. This Frankenstein rear end was a perfect candidate for a rebuild, one that would not break the bank, but one that would not use questionable salvage yard parts, either.
Some may wonder why build an 8.2 rather than an 8.5-inch 10-bolt or even a 12-bolt. Have you tried finding a 12-bolt at a swap meet lately? And as for the 8.2 vs. the 8.5 build, simply put, that's all we had under the car, so we built what we had. That's $1,500 for a rear end that should hopefully see a good 20 years of service. That's worth it. Just don't take it out to the track and try and run a 600hp engine with slicks. That's not what it was built for.