How to Determine your rear end type

[rIbBaD]

I would like to know how to determine the size and type of your rear end.

 

[jackbootedthug]

Unless you have a Hurst or a 442 you will have the 7.5" unless sombody else modified it already. On the bottom of the differential it will have 2 tabs on either side for the 7.5" or 2 large kind of like half squares or dog ears for the 8.5". There are also codes on the, i think passenger side, axel tube:
978 - 1988:
First, ALL GM 1981 to 1988 G-Bodies used a 7.5" ring gear rear axle EXCEPT: Monte Carlo SS - 7-5/8" 3.73, Olds 442 - 8.5" 3.73, T-Type and
Grand National - 8.5" 3.42 or 3.73. The 8.5" axle is the same used in the B-body (full sized) except a shorter axle tube length.
Second, the 7-5/8" axle is actually used in the mid 80's F-bodies Camaro & Firebird. This axle is the one used in the Monte Carlo SS.
Third, the '80s S and T trucks use a 7.5" ring gear axle. A short trip to the service manuals on these vehicles is all it takes to check this
information out.

The 442 and Regal axles are hard to find because 442s are rare and most T-types and Grand Nationals are not junked yet. For older Cutlasses,
the B-body 8.5" axle might fit better than the G-body axle.

RPO |--------Axle Code--------|
Ring Gear Code Ratio Conventional Limited Slip
-------------------------------------------------------
7.5 G72 2.14 2AK 2BK (aluminum brake drums)
7.5 GH2 2.29 2AH 2BH
7.5 GU1 2.41 2AJ,2AZ 2BJ,2BZ
7.5 GM8 2.56 2AA 2BA
7.5 GU2 2.73 2AB,2AX 2BB,2BX
7.5 GU4 3.08 2AC,2AV 2BC,2BV
7.5 GU5 3.23 2AD 2BD
7.625 GT4 3.73 2TF 2TH (Monte Carlo SS)
8.5 GU6 3.42 3TP 3TJ (T-Type, GN Std.)
8.5 GT4 3.73 3TG,3TE 3TR,3TX (442, GN Opt.)

8.5 GU2 2.73 8YB 8YP (B-Body)
8.5 GU4 3.08 8YD 8YR (B-Body)
8.5 GU5 3.23 8YE 8YS (B-Body)

If you don't have the codes with you to indentify if it is an 8.5" or not, I found that if you measure the distance between the very bottom
bolt for the cover and the next adjacent one (either right of left), it will be 3 ¾" for a 8.5" and 3 1/4" for a 7.5".

All 1978 and up Olds came with 7.5" 10 bolts, no 12 bolts (Chevy or Olds) whatsoever. So guys with 1978 and up G bodies had a few options:
beef up the 7.5" with better axles and a good posi unit, or find an 8.5" out of an 83-84 HO, 85-85 442, or 84 and up T-Type or GN, or go with
a Ford 9" from Currie or Moser, or buy the control arms from Southside traction and install an older 1968 or so Chevy 12 bolt or 10 bolt.

You can also decode the RPO codes from the sticker on the back of the trunck lid. If you run a search on the forum I know you can find pictures of the 2 rear ends this topic has been covered before maybe try looking in the drivetrain threads. I hope this helped.

 

[anakputa]

Thanks for the information. I have a 3.73 from a T-Type, I thought it was a 7.5 but now I am going to check tomorrow.

 

[83cutlassowner]

one thing jackbootedthug forgot to mention it that b body axle shaftes are going to have a larger bolt pattern of 5 on 5. g bodys are 4.5 on 5 lugs. width of the shafts will also be different.

 

[jrm81bu]

one thing jackbootedthug forgot to mention it that b body axle shaftes are going to have a larger bolt pattern of 5 on 5. g bodys are 4.5 on 5 lugs. width of the shafts will also be different.

G body's are 5 x 4 3/4.

 

[pontiacman469]

Not trying to highjack but jack how'd you get a red landau???Mines Maroon With a White top..And need a paint job but the white landau's killing me :x

 

[84cutspreme]

Yup g-body lugs are 5 x 4.75" also listed as 5 x 120.7mm

Here is a pic of 7.5" with tangs at bottom and flatter spots at top and bottom of cover

Here is one of 8.5 with triangles at bottom and more rounded cover

And here is a link to RPO codes and other info
http://www.g-body.org/tnt/diffs/diff-codes.php

 

[83cutlassowner]

one thing jackbootedthug forgot to mention it that b body axle shaftes are going to have a larger bolt pattern of 5 on 5. g bodys are 4.5 on 5 lugs. width of the shafts will also be different.

G body's are 5 x 4 3/4.

lol missed the 7. but yea your right 4.75 on 5