Читать книгу Chevy Differentials - Jefferson Bryant - Страница 8
ОглавлениеChevy 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 to disc brakes.
Bolt-in axles include (right to left) the axle, retainer plate, split washer shim, press-on bearing, and housing end. To remove the axle shafts, you need to remove the four bolts.
The rear cover’s shape and the number of bolts are identifying features for GM rear differentials. The round 10-bolt cover with a bulge for the ring gear identifies this axle assembly as an 8.5-inch 10-bolt. The two lugs on the lower case at the 5 and 7 o’clock positions are also identifying features. The 8.2-inch differential does not have these lugs.
7.5/7.625-Inch Units
To positively identify the Chevy 10-bolt in the 7.5/7.625-inch size, you need to measure it because it is very similar to the 8.5-inch housing. The case has a similar pair of lugs at the base of the center of the housing, which are located at 5 and 7 o’clock. However, the 7.5-inch lugs are smaller, with the outer side running at an angle and the inner side cut with a radius. The oval-shaped cover measures 8 inches by 10 inches. The distance between the lower center cover bolt and its adjacent bolts is 3¼ inches. Inside, the ring gear bolts are the same as the 8.5 corporate unit. However, the pinion shaft measures 1.438 inches. The axles are retained by a set of C-clips on the inner end of the axle shaft inside the carrier.
A pair of long flat areas on the front side of each axle tube is a clear indicator of an 8.5-inch Chevy 10-bolt.
To help you identify the 8.2-inch housing, remember that it may have an irregular-shaped cover or a round cover, but it does not have lugs as on the 8.5-inch.
Chevy 10-Bolt Models
Although the 8.5- and 8.6-inch rear axles are more than capable of handling 400 hp (and with some setups a bit more), the 10-bolt name has a bad reputation due to the inherently weaker 7.5 and 8.2 designs. Because these two sizes are so common in pre-1971 (8.2) and 1975–2002 (7.5) vehicles, the 8.5 is lumped into the same group. This design was used in all GM rear-drive cars from 1964 through 1972. The 8.2 was phased out starting in 1971; it was replaced by the 8.5-inch “corporate” 10-bolt, and was installed in everything from Camaros and Chevelles until the mid-1980s. It remained in the 1/2-ton trucks until 1999, when the 8.6 replaced it, using the same basic design.
By far, the most common 10-bolt is the 7.5/7.6, and it has been around since 1975. It was installed on small trucks and vans up to the 2005 model year. There is very little aftermarket support for this axle assembly because it couldn’t handle high-horsepower loads and therefore its performance potential was marginal. In street applications, the 7.5 is good for 350 to 400 hp with street tires and lots of wheel spin. When sticky traction bars and/or sticky tires were installed, owners found that 400 hp can quickly turn the 7.5 into shrapnel.
In the final analysis, this axle is simply too small for high-horsepower cars, and so these axles should be avoided for most muscle cars and certainly any racing applications. Although gear sets and a locking differential are available, these are only suitable for a mild street engine or possibly a dirt track car. In the world of dirt track racing, some classes require a GM 7.5-inch 10-bolt and because there is no traction on dirt, this rear works very well.
Millions of 8.2-inch axle assemblies were built and many can be found in salvage yards. And like the 7.5 axle, it has a fair amount of aftermarket support but the ring gear is too small and therefore it cannot handle much torque. If installed on a 400-hp or stronger engine, it often fails. And unfortunately, there simply isn’t enough room to install bigger axles, so it isn’t a viable option for a high-performance car. To support high torque and horsepower loads, the axle shafts need a larger diameter and spline count. Combined with the small outer bearing races, the 8.2 is limited to 28-spline axles.
For performance vehicles, the 8.2 can typically handle up to 400 hp with street tires, but that’s the limit for this axle. If you bolt on even a set of drag radials, the axles bend or break, along with having the potential for breaking the gears and carrier themselves. You can build these for performance, but if you use sticky tires, the superior traction and consequent strain from the grip will kill it quickly on the drag strip.
There are temporary fixes for the 8.2, such as a carrier girdle, but they don’t provide a reliable and suitably strong solution. When too much torque or traction is fed through the axle, it will eventually break the axle.
The 8.5- and 8.6-inch 10-bolts have larger ring-and-pinion gears, which makes an important difference. These rear axle assemblies can handle up to 400 hp. Among the Chevy 10-bolt family of axles, these provide the best performance and durability. The car versions were in production from 1971 to 1987. General Motors has been using this axle assembly in cars for 16 years and in 1/2-ton trucks for 30 years. The 2010-up Camaro uses a similar design (8.6 10-bolt) in the center section of its independent rear suspension.
The 8.5 is limited to 30-spline axles, but can withstand 1,000 hp with slicks when properly built. The factory installed the 8.5-inch 10-bolt in the Buick Grand National, and that’s the biggest claim to fame for this OEM axle. In stock form, the 8.5 can support wheel-standing launches from the turbocharged 6-cylinder. At just 3/8-inch smaller than the 8.875-inch 12-bolt differential, the 8.5-inch ring gear is strong enough for high-performance applications.
The aftermarket fully supports the 8.5. Gears of all sizes, limited-slip or Posi-Traction, lockers, and spools are offered. Affordable performance is what the 8.5 is all about. Considering the challenges of the typical 12-bolt swap for most muscle cars, when the 10-bolt units are often a bolt-in swap, the 8.5 10-bolt starts to look really good.
10-Bolt Carriers
Several differential carriers are offered for the 10-bolt axle assemblies. However, only certain gear sets are offered for the carriers, especially if you change gear ratios. Typically, 10-bolt carriers are specific to a series of gears. A 2-Series carrier holds 2.56:1 and higher gears (numerically lower) such as 2.41. These are very high gears, good for top speed, not for off-the-line performance. The 3-Series carriers are good for 2.73 and lower gears, so 3.08 and 3.73 gears work well.
Most axle assemblies have open differentials. The open differential does not have anything in the middle of the carrier between the side gears. If this were a limited-slip or Posi-Traction differential, you would see the clutch packs in the middle of the carrier and between the side gears.
The stock axles for both Chevy 10- and 12-bolt differentials use C-clips unless you have one of the rare bolt-in axle units. A small bolt in the center of the carrier retains the crossbar.
In this photo, you clearly see the clutch packs with springs, so indeed these are limited-slip differentials. A Yukon aftermarket clutch-type limited-slip differential is on the left; the GM Posi-Traction differential from a 1971 Buick Gran Sport 8.5 10-bolt is on the right. As you can see, the Yukon casting is much thicker and so are the springs.
The C-clips are not the strongest method for retaining the axle shafts; many owners convert the Chevy 10- and 12-bolt axles to a flange type, which retains the axle if it fails. To remove the C-clip, you push the axle in to allow room to snag the C-clip with a pick. Once the C-clip has been removed, the axle slides out of the housing.
The placement of the casting numbers on an 8.2-inch 10-bolt varies by year and model. When you decode these numbers you can conclusively identify your axle.
Engine torque and suspension loads are placed on the rear axle assemblies, which are also subjected to moisture, dirt, and anything the road can throw at it. You may need to clean the rear housing before you can decode the casting numbers. You can simply clean the area around the casting pad, but a power washer and some hot soapy water can work wonders for 40 years of grime.
10-Bolt Housings by the Numbers
Before you rebuild any axle, you should identify which axle you have. Once you have identified the housing, you must order the correct parts for the particular axle. The casting numbers for 10-bolt rear differentials are typically located either on the forward side of the passenger-side axle tube or on the driver’s side. These numbers are approximately 3 inches from the center section.
The two examples at right show you how to decode 10-bolt housings.
1970 axle code: COZ 01 01 G E
| COZ | Ratio |
| 01 | Month |
| 01 | Day of month |
| G | Plant |
| E | Posi-Traction source |
1971+ rear axle code: CB G 112 1 E
| CB | Ratio |
| G | Plant |
| 112 | Day of year |
| 1 | Shift |
| E | Posi-Traction source |
10-Bolt Gears by the Numbers
Gears are also “coded” with their teeth count; dividing the number of ring gear teeth by the number of the pinion gear teeth yields the ratio.
A full range of pinion gears is offered for the Chevy 10- and 12-bolt axle assemblies so you are able to select the correct gear set for your vehicle, application, and setup. These are two pinion gears for the 8.5-inch 10-bolt. The pinion on the left is part of a 4.11:1 gear set; the one on the right is a 3.08:1 pinion. You can see the dramatic difference in not only teeth but in overall diameter.
The tooth count is stamped on the head of each pinion for both the pinion and the ring gear. As you can see, 13 is the hypoid gear count for the pinion and 40 is the ring gear count. Pinion gears and ring gears are not interchangeable because they are designed for the specific (correct) mesh. Therefore, the specified pinion and ring gears must be used together.
Limited-Slip versus Open Differentials
As you may know, GM’s product name for a limited-slip differential is Posi-Traction. Most GM muscle car owners choose to install or use an axle assembly with Posi-Traction because it provides superior traction and performance compared to an open differential. A limited-slip differential senses wheel slip and directs torque to the wheel that is not slipping, so acceleration is optimized.
Two methods are used to determine if an axle has a limited-slip differential. A strong but not definitive indication that axle assembly has (or once had) Posi-Traction from the factory is a metal tag on one of the rear cover bolts that reads, “Use Limited Slip Lube Only.”
However, a hands-on mechanical test of the differential is the only certain way to determine if it is indeed a limited-slip. First, you must raise the rear of the vehicle and place the vehicle on jackstands. Set the transmission in neutral. Grab one wheel and spin it while having a helper watch the other wheel. A correctly functioning limited-slip differential spins both tires in the same direction. An open differential spins the other wheel in the opposite direction.
Although this hands-on test provides a good indication, it is not conclusive because a badly abused differential with worn clutches can function like an open carrier.
When it comes to GM muscle cars and sports cars, the 12-bolt axle has been the top high-performance axle assembly for decades. Compared to the Ford 9-inch, the 12-bolt positions the pinion gear higher on the ring gear. This reduces the load on the pinion, resulting in less parasitic loss from the friction and load.
The 12-bolt was introduced in 1964 and installed in cars and trucks until 1972. From 1972-on, General Motors installed its 10-bolt in cars and it remained an option for trucks until 1987.
This 1967 Chevy truck used a trailing-arm design with coil and leaf springs. The half-leaf spring (left) serves as an overload spring for heavy loads or trailering.
Unlike the various 10-bolts, the 12-bolt axle assembly has different components for cars and trucks. The passenger car 12-bolt has an oval-shaped differential cover, and it measures 10 × 10⅝ inches.
Trucks have a smaller inner pinion shaft (1.438 inches versus 1.675 inches) and bearing, and the pinion rides lower on the ring gear. In addition, the truck 12-bolt has an irregular shape. The early truck 12-bolts had large axle splines with only 12 splines. The differential carriers are also narrower than on the passenger car units, and they do not interchange. That does not mean that the truck units are not capable of performance builds because aftermarket 30-spline carriers and axles are available.
General Motors installed different axles for different applications. Axles for high-performance or heavy-duty applications commonly used higher spline-count axles while common passenger car axles use lower spline counts. The top axle is an 8.5-inch 10-bolt with 30 splines; the bottom axle is an 8.5-inch with 28 splines. Note the thicker head on the bottom axle where the C-clip rides. This is specific to the carrier. The carrier and axles must match.
The truck 12-bolt axles are much more affordable than the car units because they are more plentiful but these units have fewer splines so they are not as strong as the axle in the car assemblies. In addition, the trucks typically have larger axles and brakes.
Most passenger car 12-bolts used a four-bar trailing arm mounting system, with the exception of the Camaro and Nova, which used leaf springs. GM trucks from 1961 through 1967 used a two-bar trailing arm mount, while the 1968-up trucks used leaf springs. There is some crossover on the trucks, as some earlier trucks had leaves and some later trucks had the trailing arms.
All GM 12-bolts use C-clip–style axles. Aftermarket 12-bolt housings are based on the passenger car design.
General Motors used several different suspension designs in their passenger cars throughout the 1960s and 1970s. This 8.5-inch 10-bolt came from a 1971 Buick GS. The large bushings at the top of the differential housing connect to the triangulated four-bar trailing arm system that the Buick used. It is more difficult to swap these housings from car to car if they do not share the same suspension design.
Camaros, Novas, and 1968 and later trucks used leaf springs like these. The axle may be over or under the leaf, depending on the application.
C2 and C3 Corvettes (built from 1963 to 1981) used a non-standard 10-bolt design. They used an independent rear suspension with transverse leaf springs. As a result, these cars use a specialized axle housing for this suspension, and it’s not easily upgraded. You need to machine the housing to accept a 12-bolt carrier, which also requires custom axles. Essentially, the housing is machined to clear the larger gears and carrier, and it’s not a job for the novice.
The Chevy 12-bolt axle assemblies for passenger cars feature an oval cover with a diagonal indentation. This is a 1969 Chevelle 12-bolt housing.
12-Bolt Carriers
The 12-bolt carriers also use the same series-specific system as do the 10-bolts; each carrier only works with certain gear sizes. The types are 2-, 3-, and 4-Series. The 2-Series is by far the most common.
12-Bolt Housings by the Numbers
The casting numbers for the 12-bolt housings are typically found on the upper rear of the driver’s side of the center section. The casting numbers are simple to decode.
The first letter is the month of the year; A is January, B is February, and so on. The next digit is the day it was built, and the last digit is the year it was built. For example, a 12-bolt axle that was built on March 28, 1967, is C287.
On the passenger-side front tube, the stamped axle code designates either 1969-and-earlier units or 1969-and-later builds. The 1969-and-earlier codes have two letters, then a four-digit number, followed by a letter, and possibly a shift number, for which 1 is the day shift and 2 is the night shift.
And finally, a Posi-Traction number was used.
For 1969 and later, the code typically features six to eight digits, including three letters, three numbers, and sometimes an additional number and letter. The first two letters indicate the gear-ratio code, the third letter notes the build plant, and three numbers designate the build day from 001 to 365. Sometimes the shift code is stamped, and if the unit has a Posi-Traction, you see a P stamp.
Truck 12-bolts have an irregular cover with a ring gear pocket. This example is a 1967 Chevy C10. The truck housings are not as durable as the passenger car housings due to a narrower carrier and a smaller inner pinion bearing.
