The few letters and numbers stamped on a GM axle tube tell you the gear ratio, whether it left the plant with Positraction, roughly when it was built and where. The trouble is that GM reused codes year to year, and the layout changed for 1971. This decoder reads both layouts and checks the code against 601 ratio codes for 1964-72 Chevelles and 1962-79 Novas. Below it are plain tables for identifying the axle by its cover, case and casting number. If you are under the car anyway, our A-body rear drum brake shoe guide covers the brakes on the same axle.
Quick answers
- The code is on the bottom or front of the passenger-side axle tube, next to the carrier or 8 to 10 in. inboard of the backing plate (1965 and later). Early Chevy II units carry it on the carrier. [12][7]
- 1970 and earlier: ratio letters + month + day + plant. CG1025B = 3.36:1 10-bolt Positraction in a 1969 Chevelle, built October 25 at plant B. [12]
- 1971 and later: ratio letters + plant + day of year. CHC064 = 2.73:1 12-bolt Positraction in a 1972 Chevelle, built at Buffalo on day 64. [17]
- 12-bolt car axle: 12 cover bolts, 8.875 in. ring gear, 30-spline C-clip axles, 1964-72. [3][4]
- Chevy 10-bolt 8.2: smooth lower case. 8.5: two square lugs at 5 and 7 o’clock. 7.5: oval 8-5/16 x 10-9/16 in. cover. [1]
- Codes repeat across years with different ratios, so always pick the model year. [7]
How do you decode a GM rear axle code?
Type the code as stamped. Add the model year and car line if you know them. The decoder tries each GM layout and shows every reading that fits, with the source for each value.
GM Rear Axle Code Decoder
Spaces and dashes are fine.
Narrows ratio matches; codes were reused year to year.
Decodes using the ChevelleStuff, NovaResource and H-Body FAQ tables cited on this page. A code that is not in those tables is shown as format and date only. Confirm with the ID features and casting numbers below.
What do the letters and numbers in a GM axle code mean?
1970 and earlier: two or three ratio letters, the build month and day (ChevelleStuff notes the month is often stamped as a single digit [8]), the plant letter, and sometimes a shift digit and Positraction letter. Example from ChevelleStuff: CGI0629B, a 1970 3.36:1 heavy-duty Positraction 10-bolt. [16][7]
1971 and later: ratio letters, the plant letter, the three-digit day of the year, and sometimes a shift letter and Positraction letter. Example from NovaResource: GWG218DE, a 1971 Nova 3.36 Positraction 10-bolt, plant G, day 218, day shift, Eaton unit. [7]
Watch the year: axles were built ahead of the cars, so the build date can fall in the calendar year before the model year. ChevelleStuff’s 1972 examples include day-215 codes from 1971. [17] One more catch: day 64 is March 5 in a normal year but March 4 in a leap year like 1972. The decoder shows both. On 1978 and later H-body axles the fourth character is a model-year digit. [14]
| Letter | Plant, 1970 and earlier | Plant, 1971 and later | Positraction unit | Shift |
|---|---|---|---|---|
| B | Buffalo, NY (ChevelleStuff) Buick (NovaResource) | Buick (NovaResource) Buick (H-Body FAQ) | – | – |
| C | Buffalo (NovaResource) | Buffalo, NY (ChevelleStuff) Buffalo (NovaResource) Chevrolet Buffalo (H-Body FAQ) | – | – |
| D | Cadillac (NovaResource) | Cadillac (NovaResource) | Dana Positraction differential (NovaResource) | Day shift (NovaResource) Day shift (H-Body FAQ) |
| E | – | – | Eaton Positraction differential (NovaResource) Eaton (H-Body FAQ) | – |
| G | Chevrolet Gear & Axle (Detroit) (ChevelleStuff) Detroit Gear & Axle (NovaResource) | Detroit Gear & Axle (ChevelleStuff) Detroit Gear & Axle (NovaResource) Chevrolet Gear and Axle (H-Body FAQ) | Detroit G&A Positraction carrier (NovaResource) | – |
| K | McKinnon Industries (GM of Canada) (ChevelleStuff) GM of Canada (NovaResource) | McKinnon Industries (GM of Canada) (ChevelleStuff) GM of Canada (NovaResource) GM of Canada, St. Catharines (McKinnon) (H-Body FAQ) | – | – |
| M | Pontiac/Canada (NovaResource) | Pontiac/Canada (NovaResource) Pontiac Canada (H-Body FAQ) | – | – |
| N | – | – | – | Night shift (NovaResource) Night shift (H-Body FAQ) |
| O | Oldsmobile (NovaResource) | Oldsmobile (NovaResource) Oldsmobile (H-Body FAQ) | Oldsmobile Positraction carrier (NovaResource) Olds (H-Body FAQ) | – |
| P | Pontiac (NovaResource) | Pontiac (NovaResource) Pontiac (H-Body FAQ) | – | – |
| W | Warren, MI (ChevelleStuff) Warren, MI (NovaResource) | Warren, MI (ChevelleStuff) Warren, MI (NovaResource) Chevrolet Warren (H-Body FAQ) | Warren/Warner Motive (NovaResource) Borg Warner (H-Body FAQ) | – |
| 1 | – | – | – | Day shift (NovaResource) |
| 2 | – | – | – | Night shift (NovaResource) |
Sources: ChevelleStuff [12][13], NovaResource [7], H-Body FAQ [14].
How do I identify a GM 10-bolt or 12-bolt by its features?
Count the cover bolts first, then check the cover shape, the lower case and the pinion. Where two references give different measurements, both are shown.
| Axle | Ring gear | Cover | Case and lower bolts | Pinion | Axles and notes | Years | Source |
|---|---|---|---|---|---|---|---|
| GM 10-bolt 7.5 in. (7.625) | 7.5 in. | Oval, 8-5/16 x 10-9/16 in., 10 bolts | 3-1/4 in. lower center bolt to neighbor; two small cast lugs | 1.438 in. pinion shaft | C-clip | Since 1975 (cars, small trucks, vans to 2005) | [1] |
| GM 10-bolt 8.2 in. (Chevrolet) | 8.2 in. | 11 in. cover with diagonal protrusion at top / 10-5/8 in. irregular (both stated), 10 bolts | Smooth round lower case, no cast-in lugs | 1.438 in., 25 splines (Chevy Hardcore); 1.435 in. (Hemmings) | C-clip. Ring gear bolts 3/8-24 left-hand (Chevy Hardcore) vs .375 right-hand (Hemmings): conflict | Replaced by 8.5 in 1970 production cars | [1][2] |
| GM 10-bolt 8.2 in. BOP | 8.2 in. | Same external bosses/brackets as Chevy 8.2 | Same as Chevy 8.2 | Gears do not interchange with Chevy 8.2 | Bolt-in (4-bolt retainer plates), not C-clip | Buick/Olds/Pontiac | [1] |
| GM 10-bolt 8.5 in. | 8.5 in. | 11 in. round with ring-gear bulge (Chevy Hardcore); 11 x 10.25 in. oval (Hemmings), 10 bolts | Two large square cast lugs at 5 and 7 o'clock; 3-3/4 in. lower bolt spacing; 1-1/4 in. pinion nut | 1.625 in., 28 or 30 splines | C-clip (bolt-in on some 1971-72 BOP and 1970-72 Monte Carlo). Ring gear bolts 7/16-20 left-hand | From 1970 production cars to 1999 | [1][2] |
| GM 12-bolt passenger car | 8.875 in. | Oval, 10-15/16 x 10-5/8 in. (Chevy Hardcore) / 10.9375 x 10.625 in. (Hemmings), 12 bolts, inverted-V 'hat' at top | Camaro/Nova leaf-spring saddles; Chevelle/Monte Carlo upper arm mounts on carrier | 1.625 in. pinion shaft (Hemmings); 1.626 in. (Rigid Axle); Chevy Hardcore lists car inner pinion bearing 1.675 in. | 30-spline C-clip axles (all OE Positraction 12-bolts) | 1964-1972 cars | [3][4][5][6] |
| GM 12-bolt truck | 8.875 in. | Irregular-shaped cover, 10.875 x 10.875 in., 12 bolts | Not interchangeable with car 12-bolt | 1.438 in. (smaller inner pinion bearing) | Early truck units: 12 large axle splines | 1965-1987 trucks/vans | [3][4] |
What do GM carrier series numbers mean?
| Axle | Series | Ratios | Source |
|---|---|---|---|
| 12-bolt car | Type/Series 2 | 2.73 and numerically lower | [3][7] |
| 12-bolt car | Type/Series 3 | 3.08 to 3.73 | [3][7] |
| 12-bolt car | Type/Series 4 | 3.90 and up (Chevy Hardcore: 3.90-6.14; Rigid Axle: 4.10 and higher) | [3][5][7] |
| 10-bolt 8.5 | Series 2 / 3 deck heights | 2.56 and lower: 1.530 in.; 2.73 and higher: 1.720 in. | [1] |
| 10-bolt 8.2 | Deck heights | 2.73 and lower: 1.615 in.; 3.08 and higher: 2.00 in. | [1] |
| 10-bolt 7.5 | Deck heights | 3.08 and lower: 1.660 in.; 3.23 and higher: 1.950 in. | [1] |
What are the GM 10-bolt and 12-bolt casting numbers?
A housing casting number confirms the housing type, but not the gear ratio inside it.
| Car line | Years | 10-bolt | 12-bolt | Note | Source |
|---|---|---|---|---|---|
| Chevelle | 1964 | 3839390N | not listed | Housing casting number; may carry an N or NF prefix or suffix. | [8] |
| Chevelle | 1965 | 3859139 (also 3863495N, reported as relatively rare) | 3859140 | Housing casting number; may carry an N or NF prefix or suffix. | [9] |
| Chevelle | 1966 | 3875744N | 3875745N | Housing casting number; may carry an N or NF prefix or suffix. | [10] |
| Chevelle | 1967 | 3894938N | N3894939 (narrow, early model year); NF3917124 (wide, late model year) | Housing casting number; may carry an N or NF prefix or suffix. | [11] |
| Chevelle | 1968-1969 | 3917123 | 3917124 (also 3959038 in 1969) | Housing casting number; may carry an N or NF prefix or suffix. | [12] |
| Chevelle | 1970-1972 | 3969277 | 3969278 | Housing casting number; may carry an N or NF prefix or suffix. | [13] |
| Nova | (by series) | 8.2: ED32118 (2.73 and down), EDB30116 (3.08 and up); 8.5: 410409N (2.56 and down), 410408N (2.73 and up) | ED32088 (2.73 and down), 30140PM1 (3.08-3.73), EDB30174 (3.90 and up) | Nova figures are differential CARRIER casting numbers by carrier series, not housing castings. | [7] |
What ratio does my GM axle code mean, by year?
These are the full code lists the decoder uses. Open a year to see its table. Ratio codes not listed for a year were not in the source for that year, so they are left out.
Chevelle and El Camino, 1964-1972
1964 Chevelle: 20 codes
| Code | Ratio and application |
|---|---|
| LA | 3.08:1 ~ 3 or 4-speed |
| LB | 3.36:1 ~ 3 or 4-speed |
| LG | 3.08:1 ~ Positraction |
| LH | 3.36:1 ~ Positraction |
| YA | 3.08:1 ~ 3 or 4-speed |
| YB | 3.36:1 ~ 3 or 4-speed |
| YG | 3.08:1 ~ Positraction |
| YH | 3.36:1 ~ Positraction |
| ZA | 3.08:1 ~ found with Powerglide transmission |
| ZB | 3.36:1 ~ |
| ZC | 3.08:1 ~ w/Metallic Brakes |
| ZD | 3.36:1 ~ w/Metallic Brakes |
| ZE | 3.08:1 ~ Positraction w/Metallic Brakes |
| ZF | 3.36:1 ~ Positraction w/Metallic Brakes |
| ZG | 3.08:1 ~ Positraction |
| ZH | 3.36:1 ~ Positraction |
| ZJ | 3.70:1 ~ w/Overdrive |
| ZK | 3.70:1 ~ Positraction |
| ZL | 3.70:1 ~ w/Metallic Brakes |
| ZM | 3.70:1 ~ Positraction w/Metallic Brakes |
1965 Chevelle: 29 codes
| Code | Ratio and application |
|---|---|
| CA | 3.08:1 |
| CB | 3.36:1 |
| CC | 3.73:1 |
| CD | 3.07:1 ~ Positraction |
| CE | 3.08:1 ~ Positraction |
| CF | 3.31:1 ~ Positraction |
| CG | 3.36:1 ~ Positraction |
| CH | 3.70:1 ~ Positraction w/Metallic Brakes |
| CI | 3.73:1 ~ Positraction |
| CJ | 3.07:1 ~ Metallic Brakes |
| CK | 3.07:1 ~ Positraction w/Metallic Brakes |
| CL | 3.08:1 ~ Metallic Brakes |
| CM | 3.08:1 ~ Positraction w/Metallic Brakes |
| CN | 3.31:1 ~ Metallic Brakes |
| CO | 3.31:1 ~ Positraction w/Metallic Brakes |
| CP | 3.36:1 ~ Metallic Brakes |
| CQ | 3.36:1 ~ Positraction w/Metallic Brakes |
| CR | 3.70:1 ~ Positraction |
| CS | 3.70:1 ~ Positraction w/Metallic Brakes |
| CT | 3.73:1 ~ Metallic Brakes |
| CU | 3.73:1 ~ Positraction w/Metallic Brakes |
| CV | 3.70:1 ~ Overdrive |
| CW | 3.31:1 ~ "327" |
| CX | 3.07:1 ~ "327" |
| GA | 3.31:1 ~ Heavy Duty |
| GB | 2.73:1 ~ 327 w/Powerglide |
| GC | 2.73:1 ~ Positraction |
| GD | 2.73:1 ~ Metallic Brakes |
| GE | 2.73:1 ~ Positraction w/Metallic Brakes |
1966 Chevelle: 35 codes
| Code | Ratio and application |
|---|---|
| CA | 3.08:1 |
| CB | 3.36:1 |
| CC | 3.73:1 |
| CD | 3.07:1 ~ Positraction |
| CE | 3.08:1 ~ Positraction |
| CF | 3.31:1 ~ Positraction |
| CG | 3.36:1 ~ Positraction |
| CH | 3.70:1 ~ w/ Metallic Brakes |
| CI | 3.73:1 ~ Positraction |
| CJ | 3.07:1 ~ w/Metallic Brakes |
| CK | 3.07:1 ~ Positraction w/Metallic Brakes |
| CL | 3.08:1 ~ w/Metallic Brakes |
| CM | 3.08:1 ~ Positraction w/Metallic Brakes |
| CN | 3.31:1 ~ w/Metallic Brakes |
| CO | 3.31:1 ~ Positraction w/Metallic Brakes |
| CP | 3.36:1 ~ w/Metallic Brakes |
| CQ | 3.36:1 ~ Positraction w/Metallic Brakes |
| CR | 3.70:1 ~ Positraction |
| CS | 3.70:1 ~ Positraction w/Metallic Brakes |
| CT | 3.73:1 ~ w/Metallic Brakes |
| CU | 3.73:1 ~ Positraction w/Metallic Brakes |
| CV | 3.70:1 ~ Overdrive |
| CW | 3.31:1 |
| CX | 3.07:1 |
| KF | 3.55:1 ~ w/Metallic Brakes |
| KG | 3.55:1 ~ Positraction w/Metallic Brakes |
| KH | 3.55:1 ~ Positraction |
| KI | 2.73:1 ~ "396" w/Powerglide |
| KJ | 3.55:1 |
| KK | 4.10:1 ~ Positraction |
| KL | 4.10:1 ~ Positraction w/Metallic Brakes |
| KM | 4.56:1 ~ Positraction |
| KN | 4.56:1 ~ Positraction w/Metallic Brakes |
| KO | 4.88:1 ~ Positraction |
| KP | 4.88:1 ~ Positraction w/Metallic Brakes |
1967 Chevelle: 50 codes
| Code | Ratio and application |
|---|---|
| CA | 3.08:1 |
| CB | 3.36:1 |
| CC | 3.73:1 |
| CD | 3.07:1 ~ Positraction |
| CE | 3.08:1 ~ Positraction |
| CF | 3.31:1 ~ Positraction |
| CG | 3.36:1 ~ Positraction |
| CH | 2.73:1 ~ Positraction |
| CI | 3.73:1 ~ Positraction |
| CJ | 3.07:1 ~ w/Metallic Brakes |
| CK | 3.07:1 ~ Positraction w/Metallic Brakes |
| CL | 2.73:1 ~ Positraction |
| CM | 2.73:1 ~ Positraction w/Metallic Brakes |
| CN | 3.31:1 ~ w/Metallic Brakes |
| CO | 3.31:1 ~ Positraction w/Metallic Brakes |
| CR | 3.70:1 ~ Positraction |
| CS | 2.56:1 ~ w/Metallic Brakes |
| CT | 3.73:1 ~ w/Metallic Brakes |
| CU | 3.73:1 ~ Positraction w/Metallic Brakes |
| CV | 3.70:1 |
| CW | 3.31:1 |
| CX | 3.07:1 |
| CY | 3.07:1 |
| CZ | 2.73:1 |
| KA | 3.55:1 |
| KB | 3.55:1 ~ Positraction |
| KC | 2.73:1 ~ Positraction |
| KD | 2.73:1 |
| KE | 2.56:1 ~ Positraction |
| KF | 3.55:1 ~ Positraction |
| KG | 3.55:1 ~ Positraction w/Metallic Brakes |
| KH | 3.55:1 ~ w/Metallic Brakes |
| KI | 2.56:1 ~ Positraction w/Metallic Brakes |
| KJ | 3.55:1 |
| KK | 4.10:1 ~ Positraction |
| KL | 4.10:1 ~ w/Metallic Brakes |
| KM | 4.56:1 ~ Positraction |
| KN | 4.56:1 ~ w/Metallic Brakes |
| KO | 4.88:1 ~ Positraction |
| KP | 4.88:1 ~ w/Metallic Brakes |
| KQ | 2.73:1 ~ w/Metallic Brakes |
| KR | 2.73:1 ~ Positraction w/Metallic Brakes |
| KS | 3.07:1 ~ w/Metallic Brakes |
| KT | 3.07:1 ~ Positraction w/Metallic Brakes |
| KU | 3.31:1 ~ w/Metallic Brakes |
| KV | 3.31:1 ~ Positraction |
| KW | 2.73:1 ~ Positraction |
| KX | 3.07:1 ~ Positraction |
| KY | 3.31:1 |
| KZ | 3.31:1 |
1968 Chevelle: 60 codes
| Code | Ratio and application |
|---|---|
| CA | 3.08:1 |
| CB | 3.36:1 |
| CC | 3.73:1 |
| CD | 3.07:1 ~ Positraction |
| CE | 3.08:1 ~ Positraction |
| CF | 3.31:1 ~ Positraction (L35/396)* |
| CG | 3.36:1 ~ Positraction |
| CH | 2.73:1 ~ Positraction |
| CI | 3.73:1 ~ Positraction |
| CJ | 3.08:1 |
| CK | 3.07:1 |
| CL | 2.73:1 |
| CM | 2.73:1 ~ Positraction |
| CN | 3.31:1 |
| CO | 3.31:1 ~ Positraction |
| CQ | 2.56:1 |
| CS | 2.56:1 |
| CT | 3.73:1 |
| CV | 3.70:1 |
| CW | 3.31:1 |
| CX | 3.07:1 |
| CY | 3.07:1 (L35/396)* |
| CZ | 2.73:1 (L35/396)* |
| FH | 2.73:1 ~ small ring gear (Note 1) |
| FI | 2.73:1 ~ Positraction, small ring gear (Note 1) |
| FJ | 3.08:1 (Base 230/base 307)* |
| FK | 3.08:1 ~ Positraction (L22/250 & 307)* |
| FL | 3.36:1 (L73/327)* |
| FM | 3.36:1 ~ Positraction |
| FN | 3.55:1 ~ small ring gear (Note 1) |
| FO | 3.55:1 ~ Positraction, small ring gear (Note 1) |
| FP | 3.70:1 |
| FQ | 3.70:1 ~ Positraction |
| K3 | 3.55:1 ~ Positraction |
| K4 | 3.73:1 |
| K5 | 3.73:1 ~ Positraction |
| K6 | 4.10:1 ~ Positraction |
| K7 | 4.56:1 ~ Positraction |
| K8 | 4.88:1 ~ Positraction |
| KA | 3.55:1 ~ small ring gear (Note 1) |
| KB | 3.55:1 ~ Positraction, small ring gear (Note 1) |
| KC | 2.73:1 ~ Positraction, small ring gear (Note 1) |
| KD | 2.73:1 ~ large ring gear (Note 1) |
| KE | 2.56:1 ~ Positraction |
| KG | 3.55:1 ~ Positraction |
| KH | 3.55:1 |
| KI | 2.56:1 |
| KK | 4.10:1 ~ Positraction |
| KL | 4.10:1 |
| KN | 4.56:1 |
| KP | 4.88:1 |
| KQ | 2.73:1 |
| KR | 2.73:1 ~ Positraction |
| KS | 3.07:1 |
| KT | 3.07:1 ~ Positraction |
| KU | 3.31:1 |
| KV | 3.31:1 ~ Positraction |
| KX | 3.07:1 ~ Positraction |
| KY | 3.31:1 ~ Positraction |
| KZ | 3.31:1 |
1969 Chevelle: 49 codes
| Code | Ratio and application |
|---|---|
| CA | 3.08:1 ~ 10-bolt (L14/307)* |
| CB | 3.36:1 ~ 10-bolt |
| CC | 3.73:1 ~ 12-bolt |
| CD | 3.07:1 ~ 12-bolt Positraction |
| CE | 3.08:1 ~ 10-bolt Positraction |
| CF | 3.31:1 ~ 12-bolt Positraction (L34 & L35/396)* |
| CG | 3.36:1 ~ 10-bolt Positraction |
| CH | 2.73:1 ~ Positraction |
| CI | 3.73:1 ~ 12-bolt Positraction (L34/396 & L78/396)* |
| CJ | 2.56:1 |
| CK | 2.56:1 ~ Positraction |
| CL | 2.56:1 ~ 10-bolt (L65/350-2)* |
| CM | 2.56:1 ~ Positraction |
| CN | 2.56:1 ~ 12-bolt (L48/350-4 & LM1/350-4)* |
| CO | 2.56:1 ~ 10-bolt (may or may not be Positraction, it's uncertain at this point) |
| CP | 2.73:1 ~ 10-bolt (L14/307)* |
| CQ | 2.56:1 ~ 10-bolt (L65/350-2)* |
| CS | 2.56:1 ~ Positraction |
| CT | 2.73:1 |
| CW | 3.31:1 – 12-bolt (L35/396, L34/396 & L48/350-4)* |
| CX | 3.07:1 ~ 12-bolt (L35/396)* |
| CY | 3.07:1 ~ 12-bolt |
| CZ | 2.73:1 |
| FH | 2.73:1 |
| FI | 2.73:1 ~ Positraction |
| FJ | 3.08:1 ~ 10-bolt |
| FK | 3.08:1 ~ 10-bolt Positraction |
| FL | 3.36:1 ~ 10-bolt |
| FM | 3.36:1 ~ 10-bolt Positraction |
| FN | 3.55:1 |
| FO | 3.55:1 ~ Positraction |
| KA | 3.55:1~ small ring gear (Note 1) |
| KB | 3.55:1 ~ Positraction, small ring gear (Note 1) |
| KC | 2.73:1 ~ Positraction, small ring gear (Note 1) |
| KD | 2.73:1 ~ 12-bolt large ring gear (L48/350, L35/396)* |
| KE | 2.73:1 ~ Positraction, small ring gear (Note 1) |
| KF | 3.55:1 ~ 12-bolt Positraction, large ring gear (L35/396, L34/396, & L78/396)* (Note 1) |
| KG | 3.08:1 ~ 12-bolt small ring gear (Note 1) |
| KH | 3.08:1 ~ 12-bolt Positraction, small ring gear (Note 1) |
| KI | 3.36:1 ~ 10-bolt |
| KJ | 3.55:1 ~ large ring gear (Note 1) |
| KK | 4.10:1 ~ 12-bolt Positraction (L78/396)* |
| KL | 3.36:1 ~ Positraction, small ring gear (Note 1) |
| KM | 4.56:1 ~ 12-bolt Positraction |
| KN | 3.55:1 ~ small ring gear (Note 1) |
| KO | 4.88:1 ~ 12-bolt Positraction |
| KP | 3.55:1 ~ Positraction, small ring gear (Note 1) |
| KQ | 4.10:1 ~ 12-bolt Positraction, heavy duty (COPO)* |
| KX | 3.07:1 ~ 12-bolt ~ Positraction |
1970 Chevelle: 34 codes
| Code | Ratio and application |
|---|---|
| CCA | 3.08:1 ~ 10-bolt (L14/307 & L22/250)* |
| CCB | 3.36:1 ~ 10-bolt |
| CCD | 3.07:1 ~ 12-bolt Positraction |
| CCE | 3.08:1 ~ 10-bolt Positraction |
| CCF | 3.31:1 ~ 12-bolt Positraction (L34/402 & L48/350-4)* |
| CCG | 3.36:1 ~ 10-bolt Positraction |
| CCH | 2.73:1 ~ 12-bolt Positraction (L48/350-4)* |
| CCL | 2.56:1 |
| CCM | 2.56:1 ~ Positraction |
| CCN | 2.56:1 |
| CCO | 2.56:1 ~ Positraction (El Camino L34/396)* |
| CCP | 2.73:1 ~ 10-bolt (L14/307)* |
| CCW | 3.31:1 ~ 12-bolt (L34/402 & LS3/402 M.C.)* |
| CCX | 3.07:1 ~ 12-bolt |
| CGA | 2.56:1 ~ 10-bolt (L65/350-2)* |
| CGB | 2.56:1 ~ 10-bolt Heavy Duty Positraction (L65/350-2)* |
| CGC | 2.73:1 ~ Heavy Duty |
| CGD | 2.73:1 ~ Heavy Duty Positraction |
| CGE | 3.08:1 ~ 10-bolt Heavy Duty |
| CGF | 3.08:1 ~ 10-bolt Heavy Duty Positraction |
| CGG | 3.36:1 ~ 10-bolt Heavy Duty |
| CGI | 3.36:1 ~ 10-bolt Heavy Duty Positraction |
| CKC | 2.73:1 ~ Positraction |
| CKD | 2.73:1 ~ 12-bolt (L48/350-4 & LS3/402)* |
| CKF | 3.55:1 ~ 12-bolt Positraction (L78)* |
| CKJ | 3.55:1 ~ 12-bolt (L78/402)* |
| CKK | 4.10:1 ~ 12-bolt Positraction (L78/402)* |
| CRJ | 2.56:1 |
| CRK | 2.56:1 ~ 12-bolt Positraction (Monte Carlo LS5/454)* |
| CRL | 3.07:1 ~ 12-bolt |
| CRM | 3.07:1 ~ 12-bolt |
| CRU | 3.31:1 ~ 12-bolt (LS5/454 & LS6/454)* |
| CRV | 3.31:1 ~ 12-bolt Positraction (LS5/454 & LS6/454)* |
| CRW | 4.10:1 ~ 12-bolt Heavy Duty (LS6/454)* |
1971 Chevelle: 15 codes
| Code | Ratio and application |
|---|---|
| CF | 3.31:1 ~ 12-bolt Positraction (L48/350-4 & LS3/402)* |
| CH | 2.73:1 ~ 12-bolt Positraction (L48/350-4 & LS3/402)* |
| CW | 3.31:1 ~ 12-bolt (L48/350-4 & LS3/402)* |
| GA | 2.56:1 ~ 10-bolt (L65/350-2)* |
| GB | 2.56:1 ~ Positraction |
| GD | 2.73:1 ~ Positraction |
| GF | 3.08:1 ~ 10-bolt |
| GG | 3.36:1 ~ 10-bolt (base L6 & L65/350-2)* |
| GH | 2.73:1 ~ 10-bolt (L14/307)* |
| GI | 3.36:1 ~ Positraction |
| GN | 3.08:1 ~ 10-bolt (L14/307)* |
| KD | 2.73:1 ~ 12-bolt (L48/350-4 & LS3/402)* |
| RU | 3.31:1 ~ 12-bolt (LS5/454)* |
| RV | 3.31:1 ~ 12-bolt Positraction (LS5/454)* |
| RW | 4.10:1 ~ 12-bolt Positraction ** |
1972 Chevelle: 15 codes
| Code | Ratio and application |
|---|---|
| CF | 3.31:1 ~ 12-bolt Positraction (LS3/402)* |
| CH | 2.73:1 ~ 12-bolt Positraction (L48/350-4 & LS3/402)* |
| CW | 3.31:1 ~ 12-bolt (LS3/402 & L48/350-4)* |
| GA | 2.56:1 ~ 10-bolt |
| GC | 2.73:1 ~ 10-bolt (L65/350-2)* |
| GD | 2.73:1 ~ 10-bolt Positraction (L65/350-2)* |
| GF | 3.08:1 ~ 10-bolt |
| GG | 3.36:1 ~ 10-bolt (L65/350-2)* |
| GH | 2.73:1 ~ 10-bolt (L14/307)* |
| GI | 3.36:1 ~ 10-bolt Positraction |
| GN | 3.08:1 ~ 10-bolt (L14/307)* |
| KD | 2.73:1 ~ 12-bolt (L48/350-4 & LS3/402)* |
| RU | 3.31:1 ~ 12-bolt (LS5/454)* |
| RV | 3.31:1 ~ 12-bolt Positraction (LS5/454)* |
| RW | 4.10:1 ~ 12-bolt Positraction ** |
Nova and Chevy II, 1962-1979
1962 Nova: 13 codes
| Code | Ratio and application |
|---|---|
| DA | 3.36 open |
| DB | 3.08 open |
| DC | 3.55 open (4cyl – 3 speed manual) |
| DD | 3.55 posi (4cyl) |
| DE | 3.36 posi |
| DF | 3.08 posi |
| DH | 3.08 posi (6 cyl) |
| DJ | 3.08 open (metallic brakes) |
| DK | 3.08 posi (metallic brakes) |
| DL | 3.36 open (metallic brakes) |
| DM | 3.36 posi (metallic brakes) |
| DN | 3.55 open (4cyl – metallic brakes) |
| DP | 3.55 posi (4cyl – metallic brakes) |
1963 Nova: 13 codes
| Code | Ratio and application |
|---|---|
| DA | 3.36 open |
| DB | 3.08 open |
| DC | 3.55 open (4cyl – 3 speed manual) |
| DD | 3.55 posi (4cyl) |
| DE | 3.36 posi |
| DF | 3.08 posi |
| DH | 3.08 posi (6 cyl) |
| DJ | 3.08 open (metallic brakes) |
| DK | 3.08 posi (metallic brakes) |
| DL | 3.36 open (metallic brakes) |
| DM | 3.36 posi (metallic brakes) |
| DN | 3.55 open (4cyl – metallic brakes) |
| DP | 3.55 posi (4cyl – metallic brakes) |
1964 Nova: 13 codes
| Code | Ratio and application |
|---|---|
| DA | 3.36 open |
| DB | 3.08 open |
| DC | 3.55 open (4cyl – 3 speed manual) |
| DD | 3.55 posi (4cyl) |
| DE | 3.36 posi |
| DF | 3.08 posi |
| DH | 3.08 posi (6 cyl) |
| DJ | 3.08 open (metallic brakes) |
| DK | 3.08 posi (metallic brakes) |
| DL | 3.36 open (metallic brakes) |
| DM | 3.36 posi (metallic brakes) |
| DN | 3.55 open (4cyl – metallic brakes) |
| DP | 3.55 posi (4cyl – metallic brakes) |
1965 Nova: 14 codes
| Code | Ratio and application |
|---|---|
| BA | 3.08 open |
| BC | 3.36 open |
| BE | 3.08 posi |
| BF | 3.36 posi |
| BG | 3.55 posi |
| BJ | 3.08 open (metallic brakes) |
| BK | 3.08 posi (metallic brakes) |
| BL | 3.36 open (metallic brakes) |
| BM | 3.36 posi (metallic brakes) |
| BN | 3.55 open (metallic brakes) |
| BX | 2.73 open (327 – powerglide) |
| BY | 2.75 posi |
| BZ | 2.73 open (metallic brakes) |
| FD | 2.73 posi (metallic brakes) |
1966 Nova: 28 codes
| Code | Ratio and application |
|---|---|
| BA | 3.08 open 10-bolt |
| BC | 3.36 open 10-bolt |
| BE | 3.08 posi 10-bolt |
| BF | 3.36 posi 10-bolt |
| BG | 3.55 posi 10-bolt |
| BH | 3.55 open |
| BJ | 3.08 open 10-bolt (metallic brakes) |
| BK | 3.08 posi 10-bolt (metallic brakes) |
| BL | 3.36 open 10-bolt (metallic brakes) |
| BM | 3.36 posi 10-bolt (metallic brakes) |
| BN | 3.55 open 10-bolt (metallic brakes) |
| BO | 3.55 posi (metallic brakes) |
| BQ | 3.31 open 12-bolt (327 Special Hi-Per) |
| BR | 3.07 posi 12-bolt |
| BS | 3.31 posi 12-bolt |
| BT | 3.07 open 12-bolt (metallic brakes) |
| BU | 3.07 posi 12-bolt (metallic brakes) |
| BV | 3.31 open 12-bolt (metallic brakes) |
| BW | 3.31 posi 12-bolt (metallic brakes) |
| FI | 3.55 open 12-bolt (close ratio 4 speed) |
| FJ | 3.55 open 12-bolt (metallic brakes) |
| FK | 3.55 posi 12-bolt (metallic brakes) |
| FL | 3.55 posi 12-bolt |
| FM | 3.73 open |
| FN | 3.73 open (metallic brakes) |
| FO | 3.73 posi (metallic brakes) |
| FP | 3.73 posi |
| FQ | 3.07 open 12-bolt (327 Special Hi-Per) |
1967 Nova: 22 codes
| Code | Ratio and application |
|---|---|
| BA | 3.08 open 10-bolt |
| BC | 3.36 open 10-bolt |
| BE | 3.08 posi 10-bolt |
| BF | 3.36 posi 10-bolt |
| BG | 3.55 posi 10-bolt |
| BH | 3.55 open |
| BQ | 3.31 open 12-bolt (327 Special Hi-Per – A/C) |
| BR | 3.07 posi 12-bolt |
| BS | 3.31 posi 12-bolt |
| BT | 3.07 open 12-bolt (metallic brakes) |
| BU | 3.07 posi 12-bolt (metallic brakes) |
| BV | 3.31 open 12-bolt (metallic brakes) |
| BW | 3.31 posi 12-bolt (metallic brakes) |
| FI | 3.55 open 12-bolt (close ratio 4 speed) |
| FJ | 3.55 open 12-bolt (metallic brakes) |
| FK | 3.55 posi 12-bolt (metallic brakes) |
| FL | 3.55 posi 12-bolt |
| FM | 3.73 open |
| FN | 3.73 open (metallic brakes) |
| FO | 3.73 posi (metallic brakes) |
| FP | 3.73 posi |
| FQ | 3.07 open 12-bolt (327 Special Hi-Per) |
1968 Nova: 26 codes
| Code | Ratio and application |
|---|---|
| BA | 2.56 open 10-bolt |
| BB | 2.56 posi 10-bolt |
| BC | 3.36 open 10-bolt |
| BD | 3.36 posi 10-bolt |
| BI | 2.73 open |
| BK | 2.73 open |
| BL | 3.07 open 12-bolt |
| BM | 3.31 open 12-bolt |
| BN | 3.55 open |
| BP | 2.73 open |
| BQ | 2.73 posi 12-bolt |
| BR | 3.07 posi 12-bolt |
| BS | 3.31 posi 12-bolt |
| BT | 3.55 posi |
| BU | 3.73 posi 12-bolt |
| BV | 4.10 posi 12-bolt |
| BW | 4.56 posi 12-bolt |
| BX | 4.88 posi 12-bolt |
| PA | 3.08 open 10-bolt |
| PE | 3.08 posi 10-bolt |
| PH | 3.55 posi 10-bolt |
| PK | 3.55 open |
| PL | 3.55 open |
| PX | 2.73 posi 10-bolt |
| QE | 3.55 open |
| QL | 3.31 open 12-bolt |
1969 Nova: 23 codes
| Code | Ratio and application |
|---|---|
| BA | 2.56 open 10-bolt |
| BB | 2.56 posi 10-bolt |
| BC | 3.36 open 10-bolt |
| BD | 3.36 posi 10-bolt |
| BI | 2.73 open |
| BL | 3.07 open 12-bolt |
| BM | 3.31 open 12-bolt |
| BN | 3.55 open |
| BO | 3.73 open 12-bolt |
| BP | 2.73 open |
| BQ | 2.73 posi |
| BR | 3.07 posi 12-bolt |
| BS | 3.31 posi 12-bolt |
| BT | 3.55 posi |
| BU | 3.73 posi 12-bolt |
| BV | 4.10 posi 12-bolt |
| BW | 4.56 posi 12-bolt |
| BX | 4.88 posi 12-bolt |
| PA | 3.08 open 10-bolt |
| PB | 2.56 open 10-bolt |
| PC | 2.56 posi 10-bolt |
| PE | 3.08 posi 10-bolt |
| PX | 2.73 posi |
1970 Nova: 19 codes
| Code | Ratio and application |
|---|---|
| CBA | 2.56 open 10-bolt |
| CBB | 2.56 posi 10-bolt |
| CBC | 3.36 open 10-bolt |
| CBD | 3.36 posi 10-bolt |
| CBL | 3.07 open 12-bolt |
| CBM | 3.31 open 12-bolt |
| CBN | 3.55 open |
| CBP | 2.73 open 10-bolt |
| CBR | 3.07 posi 12-bolt |
| CBS | 3.31 posi 12-bolt |
| CBT | 3.55 posi |
| CBW | 4.10 posi 12-bolt |
| CPA | 3.08 open 10-bolt |
| CPE | 3.08 posi 10-bolt |
| CPI | 2.56 open 10-bolt |
| CPJ | 2.56 posi 10-bolt |
| CPO | 3.08 open 10-bolt |
| CPR | 3.08 posi 10-bolt |
| CPX | 2.73 posi 10-bolt |
1971 Nova: 14 codes
| Code | Ratio and application |
|---|---|
| BL | 3.07 open 12-bolt |
| BM | 3.31 open 12-bolt |
| BR | 3.07 posi 12-bolt |
| BS | 3.31 posi 12-bolt |
| PI | 2.56 open 10-bolt |
| PJ | 2.56 posi 10-bolt |
| PO | 3.08 open 10-bolt |
| PR | 3.08 posi 10-bolt |
| GJ | 2.56 open 10-bolt |
| GK | 2.56 posi 10-bolt |
| GR | 3.08 open 10-bolt |
| GS | 3.08 posi 10-bolt |
| GT | 3.36 open 10-bolt |
| GW | 3.36 posi 10-bolt |
1972 Nova: 13 codes
| Code | Ratio and application |
|---|---|
| GT | 3.36 open |
| JB | 2.73 open |
| JC | 3.08 open |
| JE | 3.42 open |
| JI | 3.08 open |
| JK | 3.42 open |
| JN | 2.73 posi |
| JQ | 3.08 posi |
| JS | 3.42 posi |
| JW | 3.08 posi |
| JY | 3.42 posi |
| PW | 2.73 open |
| PX | 2.73 posi |
1973 Nova: 6 codes
| Code | Ratio and application |
|---|---|
| JA | 2.73 open |
| JB | 3.08 open |
| JC | 3.42 open |
| JM | 2.73 posi |
| JN | 3.08 posi |
| JP | 3.42 posi |
1974 Nova: 6 codes
| Code | Ratio and application |
|---|---|
| JA | 2.73 open |
| JB | 3.08 open |
| JC | 3.42 open |
| JM | 2.73 posi |
| JN | 3.08 posi |
| JP | 3.42 posi |
1975 Nova: 20 codes
| Code | Ratio and application |
|---|---|
| GF | 3.42 open |
| JA | 2.56 open |
| JB | 2.73 open |
| JD | 3.08 open |
| JU | 2.56 open |
| JV | 2.73 open |
| JX | 3.08 open |
| JZ | 3.42 open |
| PA | 2.73 open |
| PC | 3.08 open |
| PH | 2.56 open |
| PT | 2.56 posi |
| PU | 2.73 posi |
| PW | 3.08 posi |
| PY | 3.42 posi |
| PZ | 3.42 open |
| 2PA | 2.73 open |
| 2PC | 3.08 open |
| 2PU | 2.73 posi |
| 2PW | 3.08 posi |
1976 Nova: 14 codes
| Code | Ratio and application |
|---|---|
| FK | 2.73 open |
| FG | 3.08 open |
| PA | 2.73 open |
| PC | 3.08 open |
| PH | 2.56 open |
| PT | 2.56 posi |
| PU | 2.73 posi |
| PW | 3.08 posi |
| PY | 3.42 posi |
| PZ | 3.42 open |
| 2PA | 2.73 open |
| 2PC | 3.08 open |
| 2PU | 2.73 posi |
| 2PW | 3.08 posi |
1977 Nova: 15 codes
| Code | Ratio and application |
|---|---|
| PA | 2.73 open |
| PC | 3.08 open |
| PH | 2.56 open |
| PT | 2.56 posi |
| PU | 2.73 posi |
| PW | 3.08 posi |
| PY | 3.42 posi |
| PZ | 3.42 open |
| TX | 3.08 open |
| TY | 2.73 open |
| TZ | 2.56 open |
| 2PA | 2.73 open |
| 2PC | 3.08 open |
| 2PU | 2.73 posi |
| 2PW | 3.08 posi |
1978 Nova: 17 codes
| Code | Ratio and application |
|---|---|
| PA | 2.73 open |
| PC | 3.08 open |
| PE | 3.42 open |
| PH | 2.56 open |
| PJ | 2.41 open |
| PS | 2.41 posi |
| PT | 2.56 posi |
| PU | 2.73 posi |
| PW | 3.08 posi |
| PY | 3.42 posi |
| 2PA | 2.73 open |
| 2PC | 3.08 open |
| 2PU | 2.73 posi |
| 2PW | 3.08 posi |
| 2TV | 2.41 open |
| 2TX | 3.08 open |
| 2TY | 2.73 open |
1979 Nova: 18 codes
| Code | Ratio and application |
|---|---|
| PA | 2.73 open |
| PC | 3.08 open |
| PE | 3.42 open |
| PH | 2.56 open |
| PJ | 2.41 open |
| PS | 2.41 posi |
| PT | 2.56 posi |
| PU | 2.73 posi |
| PW | 3.08 posi |
| PY | 3.42 posi |
| 2PA | 2.73 open |
| 2PC | 3.08 open |
| 2PU | 2.73 posi |
| 2PW | 3.08 posi |
| 2TA | 2.56 open |
| 2TB | 2.73 open |
| 2TC | 3.08 open |
| 2TE | 2.41 open |
Restoring a car where matching numbers matter? See our guide to restoring a rare muscle car. Upgrading brakes at the same time? Read what matters before buying a drum to disc kit. Planning a swap instead? Our third-gen Camaro LS-swap guide covers parts and costs. More guides are in the Car Maintenance Hub.
Frequently asked questions
Where is the GM rear axle code stamped?
ChevelleStuff places it on the bottom or front of the right (passenger-side) axle tube, either next to the carrier or about 8 to 10 inches inboard from the backing plate. NovaResource says 1965-79 codes are usually on the front of the passenger-side tube, 1964 codes on the bottom of the carrier, and 1962-63 codes on the front right side of the carrier. [12][7]
How do I read a 1970 and earlier GM axle code?
It is ratio letters, then the build month and day, then a plant letter. Example: CG1025B is ratio code CG, October 25, plant B. In the 1969 Chevelle table CG is 3.36:1 10-bolt Positraction. [12]
How do I read a 1971 and later GM axle code?
It is ratio letters, then a plant letter, then the build day of the year. Example: CHC064 is ratio CH, plant C (Buffalo), day 64. In the 1972 Chevelle table CH is a 2.73:1 12-bolt Positraction. Some codes add a shift letter (D or N, or 1 or 2) and a Positraction supplier letter. [17][7]
What does the letter at the end of a GM axle code mean?
On 1970 and earlier codes the letter after the date is the assembly plant. On later codes a letter after the shift identifies the Positraction unit supplier, for example E for Eaton. Sources disagree on some letters, so all readings are shown in Table 4. [12][7][14]
How can I tell a 10-bolt from a 12-bolt?
Count the cover bolts. A car 12-bolt has 12 cover bolts and an 8.875 in. ring gear. A 10-bolt has 10. Then use the case shape: a Chevy 8.2 has a smooth lower case with no lugs, and an 8.5 has two square lugs at 5 and 7 o'clock. [3][1]
Does the axle code prove a rear end is original?
No. It shows what the housing was built as. Housings, carriers and gears get swapped, and a rebuilt unit can carry a different ratio than the code says. To check the ratio, count driveshaft turns per wheel turn or count the ring and pinion teeth. [12]
Why does my code decode to a different ratio for another year?
GM reused the same letters with different meanings across years and car lines. Set the model year and car line in the decoder to get the right table. [8][12][7]
What we left out
- Axle spline counts for 10-bolt units, because our sources did not agree enough to state them.
- Ratio codes for Camaro, Impala, Pontiac, Buick and Oldsmobile, because we did not find a sourced table we could verify. The format and plant letters still apply, but the ratio reading will say “not in our sourced tables”.
- Ratio codes from H-body (Vega, Monza) tables, which we could not read cleanly.
Sources
Every value above comes from one of these. They are enthusiast restoration references and published specs, not GM documents, except where they reproduce GM charts. Where sources disagree, both figures are shown.
- Chevy Hardcore: 10-bolt Chevy identification guide (reputable reference)
- Hemmings: GM 10-bolt 8.5-inch differentials (reputable reference)
- Chevy Hardcore: GM 12-bolt rearend guide (reputable reference)
- Hemmings: GM 12-bolt differential tech and performance guide (reputable reference)
- Rigid Axle: GM 8.875 car rear 1964-1972 specs (aftermarket parts spec)
- Quick Performance: GM 12-bolt passenger car specs (aftermarket parts spec)
- NovaResource: Nova rear axle codes, 1962-1979 (restoration reference (author notes codes compiled from many sources, no guarantee))
- ChevelleStuff: 1964 Chevelle rear axle codes (restoration reference)
- ChevelleStuff: 1965 Chevelle rear axle codes (restoration reference)
- ChevelleStuff: 1966 Chevelle rear axle codes (restoration reference)
- ChevelleStuff: 1967 Chevelle rear axle codes (restoration reference)
- ChevelleStuff: 1969 Chevelle rear axle codes (restoration reference)
- ChevelleStuff: 1971 Chevelle rear axle codes (restoration reference)
- H-Body FAQ: the rear axle (Vega/Monza/Astre/Sunbird) (restoration reference (reproduces GM field identification chart))
- ChevelleStuff: 1968 Chevelle rear axle codes (restoration reference)
- ChevelleStuff: 1970 Chevelle rear axle codes (restoration reference)
- ChevelleStuff: 1972 Chevelle rear axle codes (restoration reference)
Photo: 1969 Chevrolet Chevelle SS 396 by MercurySable99, CC BY-SA 4.0, via Wikimedia Commons.