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I'm sure this questionis buried on here somewhere but I need to ask anyway. The rear end in my trock is a 10 bolt with a removable 3rd member. The back of axle housing is round. The tech guy at summit racing said he thought it was off of a 1957-58 truck (I was told by my dad that mine's a 55). I want to use the rear end and just upgrade. Does anyone know how to identify the rear end type and gear ratio?

Sounds like you have the stock 9 3/8" Eaton differential, which was used in open driveshaft form from '55-'62. Most common gear is 3.90, although there was a 4.11 and 3.38 available as factory options. The code is stamped on the front face of the 3rd member on the right hand side.
Here is the spot the number is located.

Do you know how to interpret the code? Mine is AF 3 02

Would the 9 3/8 Eaton be the diameter of the ring gear?

AF is the 3.90 gear, 3 02 is the date code, March 2nd. 9 3/8 is the ring gear diameter. Not much available aftermarket as far as gears/Posis/lockers go, there is a 3.38 replacement gear and I do know of a 30 spline upgrade axle shaft kit, but that's about it. Seals, bearings, and gaskets are redily available.
The 3.90 gear works very well with an overdrive transmission.

I'm looking more to make the truck a street truck. Not necessarily a street rod though it has a Nova subframe which has a 5 x 4-3/4" bolt pattern. Also, I want to convert the rear to a 5 x 4-3/4" bolt pattern. The thing is, I don't want to switch the axle housing as a matter of cost.

By the way, what type of differential is in the back end?

The diff is a standard Hotchkiss type open differential. Changing to a different bolt pattern will require redrilling the axle flanges. a good machine shop should be able to do it, but for the cost, you could probably find a differential that's already got the correct bolt pattern and perhaps a higher gear ratio. Pretty much all Chevy RWD cars and S10s ran that bolt pattern '49-'04. There are some exeptions, the '91-'96 RWD Impala uses the 5x5 truck pattern.

Maybe I'll go for the Nova Axle then. I didn't really want to go through the process of re-welding the perches for the leaf springs. If nothing else, I can build the rear end first. Unless there's a way to convert the Nova front end to a 6 lug pattern. I' been thinking about that for a couple of days now.

When measuring a rear end for an axle swap do you measure mounting face on the drums or to the axle housing flange? I want to put a Nova rear end in my truck and need to get the proper measurements.

You measure both. The wheel mounting surface to be sure you'll get the wheels where you want them, and the distance between the drums to be sure they will fit outside the springs and frame.

Generally if the wheel mounting surface is right the other will be also, but not always.

Ok, I looked online and found that a Nova is 60.25 outside width which I think is comparable to the 1955 rear end I have. Now I just need to find one to get the inside width. Thanks again.

Is there a recommended replacement differential for this open type Hotchkiss? When I do a search online it mentions Hotchkiss Suspensions. Still trying to decide whether or not to buy custom axle shafts and just use the 1955 rear end I have for my truck.

What engine are you running? the reason I ask is because the HO33 rear you have will not live long behind 300 plus horse power. Ask me how I know! The gears and the axle shafts are plenty strong, but the housing is not. I spent a lot of money rebuilding the stock axle to be left only with scrap metal.

John

Honestly, I'm thinking about a 350 with a TH400 or a 700R4. I'm also considering a modern 4.8L with a 4L60E Trans. I think that would be about 285 - 300 HP between the two motors.

Got any photos of the detroyed rear end?

Did the housing just come apart?