GM 10-bolt Identification Guide. Know What You’re Looking At

When it comes to Chevrolet rearends, it’s the 12-bolt that seems to get all the accolades. Rightfully so, because when comparing the 10 and 12-bolt units, it’s definitely the more durable of the two. That’s usually proven when used in high-performance, high-horsepower applications. But for all intents and purposes, the 10-bolt rearend is a great unit for a street/strip car, and can be stout enough to handle even the occasional abuse delivered by use at the racetrack.

10-bolt

The 7.5 rearend uses an oval cover and the housing has two small cast-in protrusions near two of the bottom cover bolts. The control arm-mounting ears on top of this 7.5 rearend denotes it is usable for any 1978-’88 GM A or G-body intermediate like a Cutlass, Monte Carlo, El Camino, Malibu, Regal, or Grand Prix.

Finding a 12-bolt that is “affordable,” is getting to be a nearly impossible endeavor. For that reason, many enthusiasts are considering the 10-bolt. But, did you know there is more than one style of 10-bolt rearend? One is the small, 7.5-inch, another is the 8.2-inch, and yet two more are the 8.5 and 8.6-inch. If you’re looking for a good.economical 10-bolt rearend to rebuild or install into your hot rod, you will need to be able to accurately identify the different units so you don’t accidentally come home with a 7.5 or 8.2-inch differential.

10-bolt

If you locate a rearend that has coil spring perches, it could be from a G, A, B-body, or El Camino.

The 8.5-inch 10-bolt rearend was used in cars and trucks over a wide range of years. It first appeared in 1970 production cars as a corporate replacement for the 8.2-inch 10-bolt rearend. It was used in different models by all GM divisions – with the exception of Cadillac. Since it was so widely used, there is a better possibility of finding one of them at a salvage yard than locating a 12-bolt. That is the impetus for why we decided to put this swap meet/junkyard identification guide together. That way, you don’t unknowingly spend money on something you really do not want.

10-bolt

While the 10-bolt rearend’s lineage can be identified by the code stamps, which are usually on the passenger’s side axle tube, there are literally hundreds of potential codes for the three different 10-bolt rearend housings. Since listing them all within this article is impossible, we’ll be focusing only on visual identification.

7.5-Inch

Although the 7.5-inch 10-bolt closely resembles the 8.5-inch housing, you can positively identify the Chevy 7.5-inch rearend by measuring it. The oval-shaped cover measures 8 5/16 inches by 10 9/16 inches. The distance between the bottom center bolt in the cover and its adjacent bolts is 3 1/4 inches. Inside, the ring-gear bolts are the same as the 8.5 corporate unit, but the pinion-shaft diameter measures 1.438 inches. Like most 10-bolts, the axles are held in place by C-clips on the inner end of the axles.

10-bolt

Date and location of manufacture are stamped in the axle tube, and this table deciphers the code.

Many times, if you locate a 10-bolt rearend, it’s usually a 7.5-inch unit. These have been around since 1975, and were installed under cars, small trucks, and vans up to the 2005 model year. If used in a daily driver or cruiser application, the 7.5-inch rearend should survive behind an engine with 350hp, if traction is limited during spirited driving. If sticky tires are used, you’ll quickly turn the 7.5-inch rear into a pile of unusable parts.

10-bolt

This is an 8.2 BOP axle. BOP stands for Buick/Oldsmobile/Pontiac. It is internally different from the 8.2 Chevy, but has the same external bosses and brackets. However, the gears will not interchange with a Chevy 8.2. The axle shafts are also held in place by four-bolt retainer plates at the outer bearing, not C-clips.

8.2-Inch

There were an untold number of 8.2-inch axle assemblies built, and although it is only marginally stronger than the 7.5-inch rearend, it does have some aftermarket support. That being said, it is not recommended for use behind engines that make a serious amount of horsepower. Again, this would be okay in a daily driver or cruiser application, but if installed behind an engine with horsepower numbers climbing into the 400 range, you can plan on an eventual failure. While a carrier-bearing girdle is available for the 8.2-inch rearend – and does give some support to the housing – it doesn’t provide a reliable and suitably strong solution.

10-bolt

The date and location of manufacture coding changed in 1971.

The easiest way to identify the 8.2-inch rearend at a glance is by the shape of the housing and the spacing between the lower bolts on the cover. The 8.2-inch 10-bolt has a smooth, round, lower-case area (no cast-in protrusions), with an 11-inch cover that has a diagonal protrusion at the top. It also uses a 10 5/8-inch irregular-shaped cover. The pinion nut should also measure 1 1/8 inches, if the OEM pinion nut is still in use.

Inside the 8.2-inch 10-bolt, the ring-gear bolts have 9/16-inch socket heads with 3/8-24 left-hand threads. The pinion diameter is 1.438 inches, and has 25 splines. Like all Chevy 10-bolts, the axles are retained by C-clips on the inner end of the axle shaft inside the carrier.

10-bolt

If you find a rearend that uses a large spring perch like this, it is from either an X or F-Body.

8.5-Inch

The 8.5-inch and 8.6-inch 10-bolt rears are stout and effective differentials that can handle more power than either the 7.5 or 8.2-inch rears. While the 8.5-inch rearend was used on vehicles up to 1999, The 8.6-inch (8.625) was used on 2000 and later-year trucks. The easiest way to tell the difference is by looking at the brakes. All 8.6-inch rears have disc brakes, and 8.5-inch units have drum brakes.

The X and F-body spring perches come in two versions: mono-leaf (left) which is shallower than the multi-leaf (right).

The 8.5-inch 10-bolt rearend is hugely popular because aftermarket carriers and gears are easily interchangeable. Originally, gear ratios ranged from 2.41 through 4.10. You will need to know that there are different series’ of differentials (series two and three). With the exception of truck differentials with 30-spline axles, and those carrying 2.41 and 2.73 gears (series two), the differentials will support all gear ratios without having to change carriers. The 8.5-inch 10-bolt also shares the same pinion shaft diameter as the more expensive 12-bolt.

10-bolt

When looking at the housing from the rear, an 8.5 will have two squared off chunks of casting hanging on each side near the bottom of the differential. An 8.2 (shown) will have the same contour as the cover. Most 8.2 factory covers also have the strange protrusion at the top.

Most 8.5-inch 10-bolt rearends have two extruded, cast-in lugs on the bottom of the differential housing at the 5:00 and 7:00 positions. The covers on the 8.5-inch rearends are often 11 inches round with a bulge on the driver’s side to accommodate the ring gear. The distance between the lower center bolt on the cover and either adjacent bolt is 3 3/4 inches. The OEM pinion nut is 1 1/4 inches.

The 8.5-inch 10-bolt rearends have ten 3/4-inch hex head bolts with 7/16-20-inch left-hand thread bolts that hold the ring gear to the carrier. The pinion shaft diameter is 1.625 inches and will have either 28 or 30 splines. Like the 8.2-inch 10-bolt, the 8.5 10-bolts use C-clips to retain the axles.

10-bolt

The 8.5 rearend will have a round cover, and part of the cover will extend rearward to make room for the ring gear. You will also notice the large, flat, cast-in protrusions at the five and seven o’clock positions. These protrusions are larger than those on the 7.5 rearend.

There is an anomaly to the “corporate” 8.5-inch axle assembly that was used in some 1971 and 1972 Buick, Oldsmobile, a few 1969 through 1972 Pontiac vehicles, as well as the 1970 through 1972 Monte Carlo. Instead of using a C-clip to hold the axle in the housing, these axle assemblies use bolt-in axles, much like an 8.2 BOP rearend. Good luck finding one of these, however, as they are very popular among performance enthusiasts.

10-bolt

Another way to tell the difference between an 8.2 and an 8.5 rearend is to pull the cover and look at the bolt holding the spider gear crosspin. If it takes a 1/2-inch wrench to remove it, it’s an 8.2. If it takes a 5/16-inch wrench to remove it, it’s an 8.5.

Different Differentials

There are several differentials for the 10-bolt rearend. That being said, limited gear sets are offered for the carriers, especially if you plan to change gear ratios. Typically, 10-bolt differentials are specific to a series of gears. A Series 2 carrier will work with 2.56 and higher gears (numerically lower) such as 2.41. These are considered highway gears that are good for top speed, not for off-the-line performance. The Series 3 carriers are good for use with 2.73 and lower gears (numerically higher), so 3.08 and 3.73 gears work well.

Unfortunately, unless you have the two units side by side, it is nearly impossible to tell them apart. There is a difference, but you can’t see it unless you set both units on their side, and then measure the distance from that surface to the face where the ring gear attaches.

Differential Differences

7.5-inch Deck Heights

3.08 and numerically lower gears = 1.660-inch

3.23 and numerically higher gears = 1.950- inch

8.2-inch Deck Heights

2.73 and numerically lower gears = 1.615-inch

3.08 and numerically higher gears = 2.00-inch

8.5-inch Deck Heights

2.56 and numerically lower gears = 1.530-inch

2.73 and numerically higher gears = 1.720-inch

Hopefully, this short list will be enough to help when you are scouring the swap meets and salvage yards, and you will have an idea of what you’re looking for, and how to identify what you find.

About the author

Randy Bolig

Randy Bolig has been working on cars, and involved in the hobby ever since he bought his first car when he was only 14 years old. His passion got him noticed by many locals, and he began to help them with their own vehicles.
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