1969 Chevelle Rear Axle Date & Ratio Codes

The rear axle identification code consists of a two letter prefix indicating the gear ratio, four digits indicating the assembly date as month/day, and a single letter suffix indicating the manufacturing location. The identification code should be stamped on the bottom or font of right axle tube either adjacent to the carrier housing or about 8" - 10" inboard from the backing plates. The particular engine RPO code is listed in parentheses as found on build sheets.

Example: CG1025B decodes to CG - 3.36:1 Positraction, 1025 - Month [Oct] and Day [25], B - Buffalo (G=Chevrolet Gear & Axle, W=Warren, K=McKinnon Industries)

Casting Numbers:
12-Bolt Casting #3917124 early model year, #3959038 has also been found with K88 cast date
10-Bolt Casting #3917123

Casting date decode here →

The casting numbers/letters on rear end housings may have either a prefix or suffix of "N" or "NF" and no reason has been found yet why they may have one or the other.

Prefix	Axle Ratio
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 (Note 2)
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

Note 1:
Just what constitutes a 'small' ring gear or 'large' ring gear is uncertain. My Chevrolet Parts and Accessories Catalog, circa 1972, only shows the gear ratio service part numbers so it is quite possible one or the other was discontinued in favor of a 'service' part. Anyone with knowledge of the differences, please contact me so I can share the information with all.

Note 2:
While the GM 'Information Kit' shows the 4.88:1 gear set as an option, Chevrolet Production Reports do not list the 4.88:1 gear set. According to a 'Salesman Accessories data catalog' there is a mention of 'Special Ratio (other than Standard, Economy, or Performance). GM's Power Team Combinations show the 4.10:1 as a Performance ratio so I would suspect the 'Special Ratio' mentioned would be a 4.56:1 or 4.88:1. I would suspect the 4.56:1 and/or 4.88:1 would be shipped separately and dealer installed.

* Engine RPO code and size following the rear axle ratio are taken from build sheets, Protect-O-Plate warranty cards, or known engine/rear end combinations and may not reflect the only engine the rear axle codes came with. Where there are no RPO/engine information following the rear axle code only means I do not have documentation to verify them. One L34 Protect-O-Plate shows engine code JC (L34) but rear end code KK (4.10:1) and H (Holley carburetor). Either the JC (L34) engine code is incorrectly stamped or the carburetor type (H) should be for a Rochester (R). Assembly Month - 1st and 2nd numbers
Number	Month	Number	Month
01	January	02	February
03	March	04	April
05	May	06	June
07	July	08	August
09	September	10	October
11	November	12	December

Assembly Day - 3rd and 4th numbers
01...31

Rear Axle Assembly Plant - 5th letter
Code	Assembly Plant
B	Buffalo, New York
G	Chevrolet Gear & Axle, Detroit, Michigan
K	McKinnon Industries (GM of Canada)
W	Warren, Michigan

Ring & Pinion Gear Tooth Combinations

8.125 Ring Gear Diameter
2.73:1 ratio	41,15
3.08:1	37,12
3.36:1	37,11
3.55:1	39,11
3.70:1	37,10
8.875 Ring Gear Diameter
2.56:1	41,16
2.73:1	41,15
3.07:1	43,14
3.31:1	43,13
3.55:1	39,11
3.73:1	41,11
4.10:1	41,10
4.56:1	41, 9
4.88:1	39, 8