Generic Rear Axle Information ~ Chevelle

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The date code on a rear end housing is simple to decode. For 1964 through 1970, It includes the month, day and year such as J257. This decodes to October 25, 1967. Month codes are A through L for January through December.

Beginning in 1971 the stamped date format changed to a 3-digit number that represents a Julian date. These dates are counted from the start of the calendar year the rear end assembly was built. An early 1971 model year Chevelle (scheduled for a late 1970 calendar year build) will have a rather large Julian date number, anywhere from approximately 240 to 365 meaning the 240th up through the 365th calendar day of 1970.

A Julian date of anywhere from 001 through approximately 239 would mean calendar year 1971 for the 1971 model year. The same would apply for the 1972 model year, a large number would mean a 1971 assembly. Just when the rear ends might be installed in a 1971 or 1972 model year Chevelle aren't known; the 239 number represents September 27 so 1971 rear ends may have been assembled in 1970 previous to that particular date and same for 1972.

The point being for 1971 and 1972 model years, dates up through late June or July could be late builds for the current model year or early model builds for the upcoming model year. Since 1970 did not use this format any large 3-digit number assembled in 1970 would be for a 1972 model year Chevelle.  There are only two ratio codes unique to 1971 and 1972 so it really doesn't make that much difference; 1971 has a GB code (Positraction 2.56:1) that 1972 does not have and 1972 has a GC code (Positraction 2.73:1) that 1971 does not have. Other than those two codes all others are shared between the two years.

The 1964 through 1969 rear end assembly and ratio codes generally followed the format above. The stamped letters and/or numbers were individually stamped and not often very deep and certainly not very "neat." Sometimes the letter "E" was stamped and sometimes not, depending on the person stamping the axle housing to note an Eaton Positraction. Since the two-letter prefix code would identify a Positraction over an open rear end, it may have been considered redundant by the person at the time. The 1970 model year saw the introduction of a 3rd letter to the ratio code. This 3rd letter is the letter "C" for Chevrolet so a 1970 ratio code might read CCF where CF is the ratio code.

The 1971 model year saw a slight change in the schema of the ratio and date stamping. The axle assembly plant code letter was moved from the last position to the 3rd position, such as CF C in the example. The two-letter ratio code and plant letter may be stamped together or may be separated by a noticeable space like "CFC" or "CF  C." There were no hard-and-fast rules on this. The date format was changed from the previous month/day format to a Julian date code of the calendar year the assembly was built beginning with the first day of the calendar year.

Since the 1971 model year was introduced in September of 1970, any 1971 Chevelles built in calendar year 1970 would have a rather large Julian date. Some early calendar year Chevelles would also have a 1970 calendar year rear end date as supplies were exhausted. The 1972 model year Chevelle would follow the same theme as 1971 model year Chevelles. A rather large Julian date would mean a late 1971 calendar year rear end assembly would have gone into early model year 1972 Chevelles.

12 bolt

10 bolt

GM 4.10:1 ring gear

* A reader recently sent me these photos of a 1966 3.07:1 ring and pinion gear set. Note the ring gear is stamped with 3862691 and not the 3863691 found in the Chevrolet Parts and Accessories Catalog.

Stamped Axle Code Location

The gear ratio code, date, and rear axle assembly plant are generally stamped into the right (passenger side) axle housing facing forward and exact locations may vary slightly. This is an example from a 1967 Chevelle and is stamped CB 1012 W.  The "CB" letters are the original ID code for a 3.36:1 ratio, 1012 is the assembly date of October 12, and the "W" indicates the plant code for Warren, MI. This identification may not always be stamped clearly and the area is often covered with years of road grime so it may take some scrubbing to reveal it.

From 1964 through 1970 the format for the rear axle information consisted of a 2-letter code for the ratio, a 4-digit number for the month/year, and a single letter for the rear axle assembly plant. If the rear end has an Eaton Positraction unit, there may also be a single letter "E" to note this.  For example, a 1969 3.55:1 Positraction rear end may be stamped something like KF 0605K E (0605 is June 5) where a 1968 may be stamped something like CY 0824B for a 3.07:1 built at the Buffalo, NY plant on August 24.

In 1971 the format changed to a 3-letter prefix for the ratio and plant, a 3-digit number being the Julian date of the CALENDAR year the rear end was built and, depending on the plant and year, there may also be an addition "1" or "2" for the plant's work shift. For example an early 1971 model year rear end may be stamped something like CHC 327 2 for 2.73:1 Positraction built at the Buffalo, NY plant on the 327th day of 1970 (November 23) during that plant's second work shift.  A 1971 rear end built after the first of the calendar year in 1971 would be something like CHC 064 2 for the same ratio but built on the 64th day of 1971 or March 5.

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