How to read the casting number and production date on a GM rear axle

The casting number is stamped into the passenger side of the rear axle housing, usually near the bottom. It is a series of letters and numbers that tells you the plant where it was made, the year of production, and sometimes the gear ratio. The format varies by decade — a 1970s casting number looks different from a 1990s one — but the location on the housing is consistent across most GM rear ends.

The production date code appears as a letter followed by a number. The letter represents the month (A for January, B for February, and so on through L for December), and the number is the last digit of the year. So "C7" means March 1967 or March 1977, depending on the decade. You need to know the vehicle's approximate year to read the date correctly.

Finding the casting number requires getting under the vehicle or removing the cover plate. Wipe away dirt and grease with a rag, then use a flashlight to read the stamped characters. Take a photo if the stamping is hard to read in person — you can zoom in on the image later.

Key Takeaways

  • The casting number is stamped on the passenger side of the axle housing and identifies the plant, year, and sometimes the gear ratio.
  • The production date code uses a letter for the month and a single digit for the year, so you must know the vehicle's decade to interpret it correctly.
  • Different GM divisions (Chevrolet, Pontiac, Oldsmobile, Buick) used different rear end codes, and the format changed significantly between the 1960s and 1990s.
  • The gear ratio is sometimes encoded in the casting number but is more reliably found by counting the ring gear teeth and pinion teeth, then dividing one by the other.
  • Online databases and forums dedicated to specific vehicle years can cross-reference your casting number to confirm the axle type and original specifications.

Decoding the casting number by decade

GM rear axles from the 1960s and early 1970s typically used a two-letter prefix followed by numbers. The first letter often indicated the division (C for Chevrolet, P for Pontiac, O for Oldsmobile), and the second letter or the numbers that followed indicated the plant and year. A casting number like "CK-3019" tells you it came from Chevrolet and was made at a specific plant, but you need a reference chart for that plant code to go further.

By the mid-1970s through the 1980s, GM shifted to a longer alphanumeric format that included more information. A number like "3955512" might appear with a date code nearby. The first digits often correspond to a part number rather than a true casting number, so you may see both stamped on the housing — one identifies the specific axle assembly, and the other identifies the casting itself.

From the 1990s onward, GM used more standardized part numbering systems. The casting number became shorter and more directly tied to the part number in GM's inventory system. This makes identification easier if you have access to a GM parts catalog or a database that cross-references part numbers to axle specifications.

Identifying the gear ratio from the casting number and ring gear

Some casting numbers encode the gear ratio directly, but not all. The most reliable method is to count the teeth on the ring gear and the pinion gear, then divide the ring gear tooth count by the pinion gear tooth count. For example, if the ring gear has 41 teeth and the pinion has 10 teeth, the ratio is 4.10. This method works on any rear end, regardless of casting number.

To count the teeth, you need to remove the cover plate and drain the fluid. Mark one tooth on the ring gear with paint or tape, then rotate the pinion until the marked tooth returns to the same position. Count how many times the pinion rotates — that number is the gear ratio. This is more labor-intensive than reading a casting number, but it is definitive.

If the casting number does include ratio information, it is usually in the last one or two digits. A casting number ending in "10" might indicate a 3.10 ratio, and one ending in "42" might indicate a 3.42 ratio. However, this convention is not universal across all GM divisions and years, so verify with a reference specific to your vehicle's year and make.

Differences between Chevrolet, Pontiac, Oldsmobile, and Buick rear ends

GM divisions used different rear axle suppliers and casting codes. Chevrolet typically used Eaton or Spicer axles and had casting numbers that started with "C" or used Chevrolet-specific part numbers. Pontiac, Oldsmobile, and Buick sometimes shared axles with Chevrolet but often had their own casting codes and part numbers to reflect division-specific engineering or sourcing.

The physical differences between these rear ends are often minimal — the same axle housing might be used across multiple divisions with only the casting number changed. However, the casting number tells you which division the vehicle was originally equipped with, which matters if you are restoring a vehicle to original specifications or trying to match parts to a specific model year.

Pontiac rear ends from the muscle car era (1960s and early 1970s) are particularly sought after by collectors, and the casting number is the primary way to verify authenticity. Similarly, Oldsmobile used some unique axle codes during the 1970s and 1980s. If you are working on a classic GM vehicle, cross-referencing the casting number with division-specific documentation is worth the effort.

Using online databases and reference materials

Several online communities and databases maintain casting number cross-references for GM rear ends. Classic car forums dedicated to specific makes — Chevrolet, Pontiac, Oldsmobile, Buick — often have sticky posts or searchable archives with casting number charts. These resources are usually maintained by enthusiasts who have compiled data from original GM documentation and years of field experience.

The most reliable sources are those that cite original GM service bulletins or parts catalogs. If a database straightforward lists casting numbers without explaining where the information came from, treat it as a starting point rather than a final answer. Cross-check against multiple sources if you are making a decision based on the identification.

Some restoration shops and machine shops that specialize in rear end work keep their own casting number references. If you are having work done on the axle, ask the shop to identify it for you — they may have access to more detailed information than what is publicly available online.

What to do if the casting number is worn or illegible

If the casting number is too worn to read, you can still identify the rear end by its physical characteristics. Measure the housing width (the distance between the mounting flanges), note the type of cover plate (removable or integral), and observe the overall shape and size. These details narrow down the possibilities significantly.

The ring gear diameter and the number of teeth also help narrow identification. A 7.5-inch ring gear is common in lighter vehicles, while an 8.5-inch or 9-inch ring gear appears in heavier trucks and performance cars. Once you know the ring gear size and tooth count, you can cross-reference that against known GM axle configurations for your vehicle's year and model.

If you still cannot identify the axle with certainty, a machine shop or rear end specialist can examine it in person. They can often identify the axle type by the internal components, the bearing configuration, and the overall design — information that is not visible from the outside but is definitive once the axle is disassembled.

Frequently Asked Questions

Where exactly is the casting number stamped on the housing?

The casting number is almost always on the passenger side of the axle housing, typically near the bottom or on the side facing down. It may be stamped directly into the metal or cast into the housing during manufacturing. You may need to remove the axle from the vehicle or get under it with a flashlight to see it clearly.

Can I identify a rear end without removing it from the vehicle?

Yes, if you can access the passenger side of the housing and see the casting number. You can also measure the housing width and ring gear diameter without removal. However, reading the casting number clearly usually requires getting under the vehicle or removing the cover plate to wipe away dirt and grease.

What does the letter in the date code represent?

The letter represents the month of production, with A for January, B for February, and so on through L for December. The number following it is the last digit of the year. So "G3" means July 1963 or July 1973, depending on the vehicle's decade.

Is the gear ratio always encoded in the casting number?

No. Some casting numbers include ratio information, but not all. The most reliable way to determine the gear ratio is to count the ring gear and pinion teeth, then divide one by the other. This method works on any rear end, regardless of whether the casting number includes ratio data.

Why does the casting number matter if I am just replacing the rear end?

If you are restoring a vehicle to original specifications or matching parts for authenticity, the casting number confirms what the vehicle was originally equipped with. It also helps you source the correct replacement if you want to maintain the original gear ratio and performance characteristics.