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GM Axle Codes Model Years 1978 - 1988

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     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.

     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.

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

     The 1964 to 1967 A-Body cars ("intermediates") rear end will bolt into a G-body Cutlass with minor love taps to the upper trailing arms. He said it is inch wider on both sides, but other than that, it is the 8.2 inch 10 bolt. Much stronger than the 7.5in. And no matter what rear end I find it will have better gears than my 2.14's!

     The 1968 to 1972 A-body differential will bolt up. Summit sells a control arm set that will properly adapt them. Be sure to get the driveshaft with the rear end, since it is well - 1/2 inch shorter. Also get the rear anti-sway bar that links the lower control arms, to help your 60 ft. times. Posi-traction units are available from GM units (clutch-type), Auburn Gear (cone-type), Eaton , in case you get find a reasonable priced differential with open gears. A number of places sell 8 1/2 in. gear sets for changing ratios.

     The 3.73 should have been available in the 442's and the Turbo Regals had 8.5 inch rear ends with 3.42 gears standard and 3.73 optional from '84 through '87.

     Earlier years had reported optional higher ratios in 7 1/2 inch units (sometimes the information in the American Musclecar Publications should be taken with a grain of salt).

     On the front of the right axle tube the second and third codes should be TJ (for open rear end) and TP (for limited slip differentials). There may be a little tag on one of the bolts on the cover indicating the need for limited differential fluid, but do not rely on that.

     The 7 1/2 inch units look similiar to the 8 1/2 inch units from the outside, but most publications show a more rounded cover for the 8 1/2 inch unit. Having had the units side by side I could barely tell the difference.

     They both are 10 bolt covers. The mounting points for the upper control arms hang out more on the smaller units, maybe a half inch !

Spohn Performance G-Body Parts

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