Differential Codes and Usage

Differential Codes and Usage

Please refer to the Differential Identification section for the location of external identification!

Identification for GM Differentials

Stamped Codes:
The two letter code is approx. 3/8" in height and is stamped into the axle tube on the inboard, passenger side (toward passenger compartment). Sometimes they are also found outboard on the passenger side (towards rear of car). There are usually more than two letters or numbers, because other manufacturing codes were stamped there.

It's normal to have to wire brush the heck out of the area before you will be able to see it, but it will be there. Typically it is covered/rusted over.

What you are looking for exactly is the two or three adjacent letters, preceded by one or more numbers, of the code that is stamped (NOT RAISED) into the axle tube.

The letters themselves can be very fine and very faint. Further, there can be flaws in the stamping dies or the application of same. What may look like one letter is actually another, so look carefully.

I recently measured the distance the code was from the right side backing plate which was 13. This one happened to be on the outboard side of the axle.

Axle ratios are identified by letter or number code. On F85, Toronado and Jetstar 88, the ratio letter codes are followed by numbers giving the date of production, identified as follows:

Full size models except Jetstar 88:
1959-64	Symbol X (circled). If no symbol, axle is non-locking.
1965-69	"L" followed by number code.
1966-69 Toronado	Letter code.
F-85 and Jetstar 88:
1961-64	Symbol X (circled). If no symbol, axle is non-locking.
1965-69	Letter codes identify both locking and non-locking.

In 1964 the F85 had another ratio marking (example: P3.08L). It is on the right rear inboard side of axle houlsing tube. P is vendor; 3.08 is ratio; L means locking axle. Axle ratio codes are located as follows:

Full size models:
1959-63	On carrier casting lower locating boss.
1964 exc Jetstar 88	On carrier casting lower locating boss.
1965-69 exc Toronado	Stamped on right rear inboard side of axle housing tube.
1966-67 Toronado	Ratio and date code stamped on the flange below fill tube boss.
1968-69 Toronado	Ratio and date code stamped on flange near RH spreader hole.
F-85 and Jetstar 88:
1961-64	On right side of housing cover.
1965-69	On right rear inboard side of axle housing tube.

Ribs on Differential:
Look at the rib(s) cast into the side of the center section. If there is only one rib it is a "B" Buick rear. If there are two converging (towards the front of the car) ribs it is a "P" Pontiac rear. The single rib rears are much weaker than the two rib versions. Among other things, the number and size of the pinion gear splines are smaller.

Internal:
On each GM differential, there is a code on the ring gear which shows the number of teeth, and therefore the ratio. Remove the rear end cover and read it, then use the table above to figure the ratio. Or do the long division.

Please refer to the Differential Codes section for internal numbers!

[ Thanks to Glenn, Karl Aune, Bill Culp, Bob Handren for this information ]

1968:
Model	Ratio	Teeth	Open	Anti-spin
Type "O" differential
except Vista Cruiser and 442	2.56	41-16	R2, R6	R3, R7
	2.78	39-14	SA, S2	SB, S3
	3.08	40-13	SC, S4	SD, S5
	3.23	42-13	SE, S6	SF, S7
	3.42	41-12	TL, S8	TM, S9
	3.91	42-11	TN, T2	TO, T3
Type "C" differential
	2.56	41-16	R8, TR	R9, TW, TH
	2.73	41-15	RA, TT	RB, TJ
	3.08	43-14	RC, T4	RD, T5
	3.31	43-13	RE, T6	RF, T7
Vista Cruiser w/400, 442	4.33	39-9		TW, TY
Vista Cruiser w/350	3.08	40-13	SM	SN
	3.23	42-13	SO	SP
	2.78	39-14	TA	TB
1969:
Model	Ratio	Teeth	Open	Anti-spin
Type "O" differential
except Vista Cruiser	2.56•	41-16	R2, R8	R3, R9
	2.56¤	41-16	R6	R7
	2.78¤	39-14	S2	S3
	3.08¤	40-13	S4	S5
	3.23¤	42-13	S6	S7
	3.42¤	41-12		S9
	2.78•	39-14	SA	SB
	3.08•	40-13	SC	SD
	3.23•	42-13	SE	SF
	3.91¤	43-11		T3
	3.42•	41-12	TM
	3.81•	43-11	TO
	4.33•	39-9	TY
Type "C" differential
	2.78•	39-14	RA	RB
	3.08•	40-13	RC	RD
	3.23•	42-13	RE	RF
	3.07¤	43-14	T4	T5
	3.31¤	43-13	T6	T7
	3.55•	39-11		TL
	3.73•	41-11		TN
	2.56¤	41-16	TR	TH
	2.73¤	41-15	TT	TJ
¤ HD brake (Police) or Disc.
• Std brake
Delmont 88 w/350	2.78	39-14	RA	RB
	3.08	40-13	RC	RD
	3.23	42-13	RE	RF
Delmont 88 w/455, Delta	2.73	41-15	QA, QK, QS	QB, QL, QT
	2.56	41-16	QC, QQ	QD, QR
	3.08	40-13	QE, QM, QY	QF, QN, QZ
98	2.73	41-15	QA, QK	QB, QL
	2.56	41-16	QC, QQ	QD, QR
	3.08	40-13	QE, QM	QF, QN
Toronado	3.07	43-14	T
Vista Cruiser	3.08	40-13	SM	SN
	3.23	42-13	SO	SP
	2.78	39-14	TA	TB
Delta 88 w/350	2.78	39-14	RA	RB
	3.08	40-13	RC	RD
	3.23	42-13	RE	RF
All Delta w/455, std. brakes, 98 optional	2.56	41-16	QC	QD
	2.93	41-14	QI	QJ
All Delta w/455, h.d. brakes, Police or disc brakes, 98 standard	2.56	41-16	QQ	QR
	2.93	41-14	QU	QV
1970:
1971:
1972:
1973:
1974:
1975:
1976:
1977:
1978:
1979:
1980:
1981:
1982:
1983:
1984:
1985:
1986:
1987:
1988:

1978 - 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.

A friend of mine just old me that 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 ½ 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 Auburn Gear works for about $295 for clutch type and $395 for the better cone type units, 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, (hopefully the 442 guys can substianiate this, and give you differential numbers to look for) 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 !

[ Thanks to Chris Fair, Bill Culp, Steve Ochs, Tom Millard, Loyd Bonecutter, Thomas Martin, Charley Buehner for this information ]