Ask Away! with Jeff Smith: 8.2 or 8.5 — How to Identify GM 10-Bolt Rear-Ends
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.
J.T.
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.
What rear end came stock in 74 k20?
About How much power can a 10 bolt with a mini spool handle? I am putting about 450 hp to the rear wheels at this time. Do I need to worry about this rear-end or should I upgrade to a 12 bolt with a full spool?
Mike, I don’t have any experience with mini-spools but perhaps the bigger question is how you intend to use the car this mini-spool is in. If this is strictly a drag car, I would update to the 12-bolt and a full dedicated spool. If it is used on the street, spools are not a good idea because turning corners requires the inner tire to travel a shorter distance. With a spool, this drastically increases the twisting motion imparted to the inboard axle. This will eventually fail the axle or the splines into the spool. The other problem with a mini-spool is that the load is now concentrated on the cross-pin. Eventually this will elongate the hole in the carrier. So if you use a mini-spool, pay close attention to the mounting points for the pin and the pin itself. I’ve seen situations where the pin elongated the hole and then could not be removed. To get the rear apart requires torching the pin!
Good information, I pulled a 10 bolt with 3:55 limited slip from my ’66 GTO and raced it in my ’67 GTO for years with no problems. But I didn’t run the big slicks and the tracks didn’t have as much bite as today. I recommend calculating the overall torque, plus considering the cars weight, trans & gearing. Somewhere I once heard you could go by the “ears” on the case at the 5 & 7 o’clock position. Not sure how accurate that is? I have a ’69 firebird 350 V8 & a ’81 trans am turbo 301 V8 both auto trans, how much difference are these 10 bolts compared to a ’67 GTO other than maybe overall width?
I have a rear diff with JV G295 1 G
I cannot get this specifically identified.
Any ideas would be appreciated.
I have a 10 bolt 8.2 or 8.5 inch rear. From what I am told it is from a 1972 Chevy Caprice. It has a 11 inch rounded cover with bolt spacing from bottom bolt to next bolt about 3 1/4 inches. It has 2 square lugs on bottom of casting. I need to know the gear ratio and the code stamped on the front side passenger axial tube is KC G201 1 4 . This unit is used in my all custom truck I have built but I need a taller gear some where to lower my high way RPM’s. Any help would be much appreciated.
The KC code identifies it as a 1974 rear axle assembly with a 3.08:1 rear gear ratio. You didn’t identify the number of bolts on the cover. It could be a 10- or 12-bolt. We will assume it is a 10-bolt – but that makes it a plenty strong 8.5-inch ring gear diameter – not the 8.2-inch. There is only one ratio taller than what you already have – a 2.73:1. Another way to reduce your highway rpm would be to go to a taller rear tire – or to use an overdrive automatic transmission like the 700-R4 or the 200-4R.
I have a 65 Chevy pick up truck, but not sure of the rear end that’s in it. There is an 8 then underneath if it has CFD. To the left there is a picture of a clock that reads 10 o clock. Does anyone know what kind of rear end I have!
Thanks in advance,
Jeanne
I am pulling my hair out trying to identify my rear ratio. I don’t see any stamps between Chunk and drum. On the chunk, iread GM19 on one side and N3894859 on the other. This is in my 67 Nova2, but doubt its the one that came with the car. Please help me here.
Can try this trick. Tape a piece of string to the drive shaft and remember the location of the taped end. Rotate the back tire(s) one full revolution. Count the number of “winds” of the string, including any partial wind ups and that is your ratio. I.e. if you have three and half revolutions of the driveshaft for one revolution of the tire, you have 3:50 to 1 ratio, etc.
I have a 1968 Chevelle Malibu 327 , automatic 350 transmission and a 10 bolt rear end, where do I find the numbers to indicate my gear ratios?
hi jeff I have a 10 bolt rear 8.5 it has disc brakes with leaf spring mounts like on a camero it measures 42.7/16 between the springs centers it also has a flat flange 4 bolt the tube number is meg72811 and the casting number is 1249618 b it has 2.56 gear ratio I can’t find anything about what it came out of. it has 11 inch rotors with big calipers thanks for any info
Just would like some opinions on this, I recently bought a 1 owner 72 El Camino custom the owner ordered it for racing only, he ordered it with no ac, no power brakes, 4 speed close ration Saginaw and posi-traction 10 bolt 8.5, the axle code on passenger side housing is G I C 080 1W, decoding the vin number tells me this is a 3.36.1 ratio and can handle up to 1000 hp to the rear wheels ?
I have a 1970 nova originally a 307 3 spd with a ten bolt rear 308 gears. How many spline and 8.2 or 8.5 ?
I have a 72 Monte Carlo and Built the motor a little. The axle code says it is a 273:1 Open. The cover is 10 bolt, has a raised section on the left side of it and the housing at 5 and 7 have pointed small lugs not the square ones. The info I get is it should be 8.2 but the description says 7.5 or 7.6. what you think because I would like to put 373:1 in it
Thank You
Tony
Tony, from my research it would be a 8.2-inch 10-bolt. The Monte Carlos did not get the corporate 8.5 until 1973. To my knowledge, GM did not use a 7.5 or 7.6 housing in those cars. If you are considering a limited slip, you will need to step up to a 3-series gear ratio because I don’t think anyone offers a limited slip differential for the 2-series gears. Finding a 8.2 factory limited slip for a 2-series gear might be very difficult.
We just purchased a chev impala 2015 Body style: LTZ 4 door sedan w/2LZ the engine is a 3.6L V6. I have been reading that there was a 9th and 10 generation to this vehicle in 2015. How do I tell which generation I have. Not all accessories are compatible with both. The paperwork shows no indication of the generation. I have looked up the vin hoping to find out more but have run into a dead end. Thanks in advance Jeff any help you can give is appreciated.
I have a 10 bolt rear with bolt in axles for my 1970 camaro, It has the 2 lugs on the bottom of the diff so I believe its a 8.5 with the cover sticking out for the ring gear. It has 4:10 Richmond gears. I found the numbers on it and can you tell me more about what it came out of? Axel tube=PXG71151 on the center part of the diff it has D201(or a 7) and where the drivers side tube is welded to the center part it has 1253. Thanks Jeff.
I have a gm axle with the stenciled code pj g91931 and a casting no of 1254333. It is currently under a 1979 model camaro and it has apparently been modified and I would like to know what it originally was put in. It is a 2 in front of the pj g91931.
Hi This is Jim
I have a Buick rear end one of the tubes there are some numbers SD B109 2 . THE CASE 1235197. It appears to me 71-72 8.5 . I think it shows a 308 posi . It has( A )on it three different spots and one X , The cover has numbers on it also 1234496. The ribs on the side or parallel. The cover has two notches in it. Could not find a lot about the rear end especially the SD
See what you can find
THANKS JIM
it’s a 10 bolt rear end
THANKS JIM G