Corvette Restoration Tip: Understanding Numbers & Codes
This Corvette Restoration Tip is brought to you by Zip Corvette and can be found in Zip’s free Corvette Parts & Accessories Catalogs – Request one online today.
Nearly all of the major parts found in 1968-1982 Corvettes contain a part number and date code. Developing a better understanding of these numbers and codes and how they relate to one another will enhance your enjoyment of the hobby.
Corvette Engine Stamp
The logical starting point for the analysis of part numbers and date codes on any Corvette is the car’s vehicle identification number (VIN). For all vintage Corvettes the first several characters of the VIN indicate the model year, body style, and assembly plant and the latter characters are the sequential number of each particular vehicle. Beginning in 1972 the fifth character in the VIN indicated which engine the car originally came with. The VIN was stamped into a rectangular metal plate that was riveted to the windshield support frame at the driver side base of the windshield on all 1969-82 Corvettes. Every car’s VIN was also stamped into the chassis in at least one place. For most C3s it was stamped into the chassis on top of the side rail on the driver side in both the area beneath the door and the area above the rear wheel. For all C3s the latter half of the VIN, often called the VIN derivative, was stamped into the car’s original engine block. This stamping was on a flat pad just above the water pump mount area on the passenger side. It also was stamped into the car’s original transmission case.
One way to help verify whether a specific engine or transmission is original to a given car is to compare the car’s VIN to the VIN derivative in the stamping on the block or case. A stamping that does not match the car’s VIN almost certainly means the engine or transmission is not original to that car. A stamping that does match the latter half of the VIN usually means the assemblies are original but even matching numbers does not guarantee this because of a practice commonly referred to as re-stamping. In order to make a non-original engine or transmission appear like the original some people will stamp the correct numbers into the parts. They accomplish this by either machining the existing numbers off or starting with a block or case that never had numbers stamped in to begin with. Evaluating the originality of engine and transmission stampings is a science unto itself and if matching numbers are important to you it is best to enlist the services of an expert.
In addition to the VIN derivative, original Corvette engines also have what’s called an assembly sequence. This is stamped into the block adjacent to the VIN sequence. The assembly sequence indicates the engine’s manufacturing plant, date of assembly, horsepower and application. As with the VIN derivative stamping, the engine assembly stamping can go a long way in helping you determine whether a given engine is original to a particular car. But as with the VIN stamping, engine assembly stampings can be “re-stamped” so just having the correct letters and numbers does not guarantee that the engine is in fact original.
Production records for all Corvettes built in the Bowling Green assembly plant still exist but precise assembly dates for C3s assembled in the St. Louis plant are not known. You can however, determine their approximate assembly day with a fair degree of accuracy. For nearly all months of production the final serial number car assembled on the last working day of the month is known and therefore the total production for each month is known. Using this information and your car’s VIN you can determine which day your car was built. For example, if the final VIN for February was 11,000 and the final VIN for March was 12,600 and your car is VIN 11,786, then you know your car was made in March. You also know they made 1,600 cars in March. Divide that number by the total number of working days in the month and you have approximately how many cars were produced each working day and the VIN range for each of those days. Plug your VIN in and you know which day your car was made – give or take a day or two for slight irregularities in daily output.
Once you have the assembly date for you car in-hand you can evaluate the correctness of all of its dated components. All component manufacturing dates have to precede the final assembly of the car and the generally accepted rule of thumb is that they should not precede it by more than 6 months. For example, if your car was assembled on May 8th all of its dated components, such as the alternator, radiator, wheels, engine, differential, transmission and body glass, will be dated earlier than May 8th but no later than about November 8th of the previous year if in fact those components are original. Reading manufacturing dates on most components is pretty straightforward. Typically, a single number represents the year, a letter represents the month with A being January, B being February, and so on, and one or two additional numbers represent the day. For example, an alternator dated 9B21 was made February 9, 1969. In addition to reading date codes it is also important to evaluate the part numbers on major components. All major cast parts, including the engine block, cylinder heads, intake and exhaust manifolds, and transmission case, have a casting number. Most other significant parts, such as the radiator, alternator, starter, distributor, carburetor and wheels, have either a stamped-in part number or a part number tag.
Upon reading this restoration tip brought to you by Zip Corvette, which can also be found in our free Corvette Parts Restoration Catalogs, you may have many questions about the numbers and codes found on your Corvette. If that is the case our Corvette Number & Paint Guides are exactly what you are in need of. Each guide incorporates materials from the GM archives that has been compiled to offer the most authoritative reference available for finding, decoding and verifying the correct part and casting numbers of every engine and drivetrain component for 1955-1982 Corvettes and other Chevrolet models. Each book is loaded with photographs, blueprints, technical bulletins and build sheets all of which will help you easily determine exactly what your Corvette is/was equipped with.
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@Jimmy C. Phillips
The first number (1) is the country of origin. The second (G) is General Motors, the third (1) is make chevrolet, the fourth (yy) is corvette, fifth is (3) for convertible, sixth is (2) for automatic trans, seventh is (5) for the LT4 engine, the eighth is (O) varies, the ninth is (T) is the year manufactured 1996, the tenth is (5) Bowling Green Kentucky, and the final sequence of numbers is the production number beginning with 100001. Hope this answers part of your question.
I have a 1979 Corvette. I was told the engine was the original. VIN#1Z8789S424955 Engine is stamped ZAJ. Is the engine a 350-195hp?
Nick,
The engine plant abbreviations, obviously differed depending on the letter designation given. “F” indicated Flint, Michigan. “T” indicates: Tonowanda, New York. Sounds like you do have a Corvette engine!
Alan,
The VIN# clearly marks this Corvette as a year 1979. “ZAJ” indicates that this is a 350-195hp, which also states it as an Automatic transmission with California Emissions!
I’m buying a 1977 Corvette. I was told it’s a number matching car is there a book for this?
Eddie,
The best book to decipher numbers and codes for a Corvette is this Corvette by the Numbers Book.