I purchased a ’68 Camaro basket case along with a pile of parts including what appears to be a 10-bolt rear-end that was not under the car. How do I identify this as either an 8.2-inch or perhaps an 8.5-inch 10-bolt? I’ve heard those later 10-bolts are stronger, but I don’t know much about them. Thanks.
Jeff Smith: When Chevrolet built the first Camaros in 1967, they were already in production of both 10- and 12-bolt rear-end housings. The 10-bolt was used in all the low performance applications. This assembly used an 8.2-inch ring gear diameter and 28-spline axles for Camaros, Chevelles, Impalas and even the early Chevy II’s. The 12-bolt was the performance axle assembly reserved for high output small-block and all the big block cars employing a larger 8.875-inch diameter ring gear and 30-spline axles. These two rear axle combinations continued from 1965 through 1970 even with the coming of the second generation Camaro in 1970.
Then in 1971, GM corporate decided they didn’t need two different axle housings so they merged the 10-bolt with the 12-bolt. The new corporate 10-bolt started with a medium, 8.5-inch ring gear diameter but retained the 12-bolt’s pinion shaft diameter of 1.625-inches to give it strength. As with all GM Salisbury rear axle assemblies, the number of bolts, in this case 10, refers to both the ring gear bolt count and the number of bolts retaining the rear cover. While many believe that the later corporate 10-bolt is as weak as the earlier 8.2, this is not the case. We know a Super Street drag racer who continues to this day running a mid-8-second, second generation Camaro that uses a near-stock configured 8.5-inch 10-bolt. So that should indicate that this rear-end assembly is more than durable enough for a very strong street car.
So let’s look at how to identify your 10-bolt. Since the housing is bare, with no gears or components, let’s first look at the housing itself. First, we’ll assume that it is a 10-bolt with leaf spring mounts welded to the housing. That identifies it at least as potentially intended for a Camaro or a last generation (1972-75) Nova. So the first thing you can do is to measure the distance between the centerline of both leaf spring perches. If the housing measures 45 3/8-inch between the centerlines of the leaf spring perches, then this is a GM corporate 8.5-inch 10-bolt for a second generation Camaro. If, on the other hand, the rear-end measures only 42 7/16-inches, then we have to do some additional investigating. The reason for this is that the early 8.2-inch 10-bolts for the 1968-69 Camaros were this width, but this is also the spec of a later corporate 8.5-inch 10-bolt out of a 1972-75 Nova. This is a somewhat common swap for early Camaros when a 12-bolt is out of the budget range. You can add strength by bolting in this corporate 8.5-inch 10-bolt.
So let’s assume that the 10-bolt you have has the narrower 42 7/16-inch leaf spring centerline spec. First, look at the casting number of the center section. If it is an early 8.2-inch 10-bolt Camaro housing, the casting number will be 3894859NF. For the 1972-75 Camaro style model 8.5-inch 10-bolt casting numbers we’ve found a 410409N and a 410408N. These differ based on the gear ratio. That may be the same number used in Novas, but we’re not clear on that.
Another clue to the rear-end’s heritage is located on the passenger side tube as code stamps in the tube itself. There are literally hundreds of these codes for the three different 10-bolt rear-end housings we’re discussing here, so it’s beyond the scope of this answer to list all of them. But to give you an example, if the axle housing were an early Camaro 8.2 10-bolt, a “PA” stamp in the leading edge of the passenger side axle tube would represent a 3.08:1 rear gear. So you might be able to track down your rear-end housing using this code. A year or so ago, a friend asked for help with a similar problem. He had 10-bolt of unknown origin and needed to verify what it was. We measured the housing width and also the center-center distance of the leaf springs and then ran the code. The code was a 2PYG number which we deciphered as an 8.5-inch 10-bolt with a 3.42:1 gear and posi-traction from a 1975 Camaro. I have a 10-bolt stamped “GZ”, which identifies it as a 2.73:1 open, 8.5-inch rear-end that came out of a 1972 Camaro. So with a little investigating (and a Sherlock Holmes hat), you should be able to determine the exact origins of your housing.