Special Tool
Illustrates the use of a special tool that may be required or can make the job easier (caption with photo explains further).
Performance Tip
Indicates a procedure or modification that can improve performance. The step most often applies to high-performance or racing engines.
Critical Inspection
Indicates that a component must be inspected to ensure proper operation of the engine.
Precision Measurement
Illustrates a precision measurement or adjustment that is required at this point in the rebuild.
Professional Mechanic Tip
Illustrates a step in the rebuild that non-professionals may not know. It may illustrate a shortcut or a trick to improve reliability, prevent component damage, etc.
Documentation Required
Illustrates a point in the rebuild where the reader should write down a particular measurement, size, part number, etc. for later reference or photograph a part, area, or system of the vehicle for future reference.
Tech Tip
Tech Tips provide brief coverage of important subject matter that doesn’t naturally fall into the text or step-by-step procedures of a chapter. Tech Tips contain valuable hints, important info, or outstanding products that professionals have discovered after years of work. These will add to your understanding of the process, and help you get the most power, economy, and reliability from your engine.
Chevy 10- and 12-bolt axle assemblies have been standard equipment on GM passenger cars, muscle cars, and trucks for decades. The rugged, reliable, and efficient Chevy 12-bolt has established itself as the preeminent rear differential for GM muscle cars since its debut in 1965. However, the smaller 10-bolt unfairly gained the reputation as a weak and inadequate rear end for high-performance applications. But there are several models in the 10-bolt line-up. The smaller 7.5- and 8.2-inch 10-bolt rear axles can’t transmit horsepower loads in excess of 400 hp. However, the 8.5- and 8.6-inch 10-bolts are extremely stout and effective rear differentials that can transmit up to 1,000 hp to the rear wheels.
The GM 10-bolt rear end is quite possibly the most misunderstood and undervalued rear differential ever created. Even though it has been used in every major GM rear-wheel-drive platform, the 10-bolt has a bad reputation for being a low-performance unit. Nothing could be further from the truth. The 10-bolt can handle just about anything you throw at it, as long as you use the right axle, either the 8.5- or 8.6-inch. That is the great caveat; there are four sizes of 10-bolt GM rear ends: 7.5/7.625-, 8.2-, 8.5-, and 8.6-inch. These sizes refer to the diameter of the ring gear, and the one you use makes a big difference in the performance potential. The 8.5- and 8.6-inch provide superior performance and have a larger ring and pinion gear surface to handle high horsepower. Also, these surfaces run cooler because of their sheer size.
You need to be able to accurately identify the GM 10-bolt. Therefore, you need be able to choose the more desirable 8.5- or 8.6-inch and avoid the smaller 7.5/7.625- and 8.2-inch units. Identifying the 10-bolt axle is easy. The nomenclature actually refers to the number of ring gear bolts. The outer cover matches; 10 bolts hold the cover onto the housing.
This is the Moser Engineering 12-bolt axle assembly. As you can see, the Chevy 12-bolt differential is one stout axle, and it was the rear axle of choice for GM muscle cars and many GM competition cars. Big-block Chevelles, Camaros, and other GM high-performance vehicles were fitted with the 12-bolt limited-slip axle to maximize torque transfer and traction. (Photo Courtesy Moser Engineering)
This ring-and-pinion gear has suffered catastrophic failure. Be sure the mesh is correct and that the installed parts are correct so you don’t destroy components. If you take off the center section cover and discover this kind of damage, you need to identify the cause so you don’t repeat this type of failure.
8.2-Inch Units
The key to identifying the 8.2 is the shape of the housing and the spacing between the lower bolts on the cover. The 8.2 has a smooth, round lower case area, with an 11-inch cover that has a diagonal indentation at the top or a 10⅝-inch irregular-shaped cover. The pinion nut should measure 1⅛ inches, as long as it is the OEM pinion nut.
Inside the 8.2, the ring gear bolts have 9/16-inch socket heads with 3/8-24 threads. The pinion diameter is 1.438 inches with 25 splines. The axles are retained by a set of C-clips on the inner end of the axle shaft inside the carrier.
8.5-Inch Units
Most 8.5-inch 10-bolts have two lugs on the bottom of the housing at the 5 and 7 o’clock positions. These should be square blocks, each with the outer side 90 degrees (vertical) to the road and the bottom-side surface horizontal to the road. The covers are often 11 inches round with a bulge on the driver’s side for the ring gear or a 10⅝-inch irregular shape with the same bulge. The distance between the lower cover bolt and either adjacent bolt is 3¾ inches. The pinion nut is 1¼ inches.
The 8.5-inch differentials have 10¾-inch hex head bolts with 7/16-20-inch left-hand thread or reverse-thread bolts that hold the ring gear to the carrier. The pinion shaft diameter is 1.625 inches with 28 or 30 splines, which is the same as the GM 12-bolt design. Most 8.5 10-bolts are C-clip axles, so a set of C-clips retains the inner end of the axle shaft inside the carrier.
A variant of this axle assembly was used in 1971–1972 Buick GSs and Skylarks, Oldsmobile Cutlasses, and some 1969–1972 Pontiac Grand Prixs, as well as the 1970–1972 Monte Carlos. These axle assemblies had bolt-in axles and were used sporadically in A-Body wagons as well. These are highly sought after, and as such, are hard to find. In this version, the axles bolt to the housing ends just as on a Ford 8- or 9-inch. This means that in the event of an axle break, the wheel stays on the car.
Buick and Oldsmobile bolt-in axles mount at the bearing flanges on the housing ends. They retain the axle shafts in the event of a failure. The four bolts that hold the drum back plate on also retain the flange. Note that this axle has been converted