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How to Decode the GM Engine Block

by Sarah Shelton

Every GM engine is stamped with an eight-digit code. When you translate this information, you can find out when and where the engine was made. In addition, the engine suffix can be cross-referenced to verify the year and body style of the vehicle the engine was built for. The engine code is different than the longer VIN number, which includes information on the model, body style and assembly plant.

Locate the engine block code. This series of seven to eight digits begins with a letter and is stamped directly on the engine. Look behind the distributor on the passenger side of the block if it is a six- cylinder GM motor. On a small block GM V8, it is stamped in front of the cylinder head on the passenger side. The code is stamped above the timing chain cover on big block V8 engines.

Wipe any grease or dirt off the code to make sure you read each digit correctly. Check that you can see the entire engine block stamp.

Look at the first digit, which will be a capital letter. This letter refers to the plant that manufactured the engine. There are seven different GM plant codes: F for Flint, H for Hydramatic, K for St. Catherines, Ontario, M for GM of Mexico, S for Saginaw Service, T for Tonawanda and V for Flint.

Read the next four digits in the code, which will be all numbers. This refers to the date that the engine was cast. The first two numbers are the month, and the last two are the day. For example, if the numbers read 0107, then the engine was cast on January 7.

Look at the remaining portion of the code. This will be two or three digits, and could include both numerals and letters; typically it is three capital letters. Find your suffix in a GM engine code reference. For example, if your code is "CMP," then the reference indicates this is a 305 engine with 240 horsepower. It was built for a 1991 Pontiac Firebird and included the L98 automatic transmission and tuned port injection. If there is more than one entry for your suffix code, then use the engine block casting date code to identify the year of the engine and identify the correct suffix entry.

About the Author

Getting hands dirty is just part of the fun for Sarah Shelton, who draws on personal experience to write home and garden, automotive and travel articles. Her pieces have appeared on ConsumerSearch.com, USA TODAY, Dremel.com and other websites. Shelton received a bachelor's degree from Fort Lewis College in Durango, Colorado and currently lives in southern Oregon.

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