Quick Answer
To identify a small block Chevy, I read two numbers. The block casting number is a 7 or 8 digit number cast into the upper rear bellhousing flange on the driver side. It identifies the raw block (displacement, year range, and main cap configuration). The engine suffix code is a 3 letter code stamped on the front deck pad ahead of the passenger side cylinder head. It identifies the original vehicle application, horsepower, and original carburetion.
Example: casting 3970010 plus suffix CMJ = a 1974 350 cubic inch V8, four bolt main caps, 160 horsepower police spec.
Step 1: Find and read the block casting number
The casting number is cast into the metal when the block is poured at the foundry, so it cannot be altered or worn off. Look on the flat shelf at the upper rear of the engine, right above the starter and just below where the cylinder head mates to the deck on the driver side. Clean off grease and paint to read the digits.
Common small block Chevy casting numbers and what they mean:
- 3970010: 350 cubic inch, 1969 to 1980. Available in two bolt and four bolt main cap versions. The most common SBC block ever produced.
- 3970014: 350 cubic inch, 1969 to 1979. Truck and passenger car.
- 14010207: 350 cubic inch, 1980 to 1985. Lower deck height.
- 14010202 / 14010203: 305 cubic inch, 1980 to 1985.
- 3914678: 327 cubic inch, 1968.
- 3956618: 307 cubic inch, 1968 to 1973.
- 14088526 / 14088548: 350 cubic inch four bolt, 1986 plus, one piece rear main seal.
- 10066034: 350 cubic inch Mexican block (Goodwrench), very thick castings.
The casting number tells me the raw displacement and approximate year range, but it does not tell me whether the block has two or four bolt mains, what crankshaft is inside, or what compression ratio. For that I need the suffix code.
Step 2: Find and read the engine suffix code
The suffix code is cold stamped (not cast) into a small machined pad on the front of the block, directly ahead of the passenger side cylinder head and just below the water pump. The pad is roughly one inch tall by three inches wide.
The stamping reads as two parts:
- Assembly date and plant prefix: A letter for the engine plant (T = Tonawanda, V = Flint, F = Flint, K = St. Catharines Canada) followed by four digits for the month and day.
- Suffix code: Two or three letters identifying the original application, horsepower, and carburetor.
Common small block suffix codes:
- CMJ: 1974 350, four bolt main, 160 horsepower.
- CLA: 1971 350, two bolt main, 245 horsepower, Corvette.
- CTL: 1970 350, four bolt main, LT-1, 370 horsepower.
- HZ: 1968 to 1969 327, 350 horsepower, Corvette.
- CKD: 1973 350, two bolt main, 145 horsepower, Camaro.
- CHC: 1970 350, two bolt main, Z/28 LT-1 350 horsepower.
The suffix code is the key to numbers matching a restoration project. A casting 3970010 is common. A casting 3970010 with the matching CTL suffix code in the correct date range, in a 1970 Corvette, is rare and significantly more valuable.
Step 3: Confirm main cap count
Many small blocks come in both two bolt and four bolt main cap configurations. The four bolt block is desirable for performance because the bottom end is stronger.
- Drop the oil pan or look up at the bottom end from below.
- Count the bolts holding each main bearing cap. Two bolts = two bolt main. Four bolts = four bolt main.
- Only the three center main caps are four bolt on a true four bolt block. The two outer caps stay two bolt.
Step 4: Separate the 350 from the 305 visually
Both engines look almost identical from the outside. The only reliable visual difference is the casting number and the cylinder head intake port size.
- 305 cubic inch: 3.736 inch bore, 3.48 inch stroke. Small intake ports for emissions and economy.
- 350 cubic inch: 4.00 inch bore, 3.48 inch stroke. Larger intake ports.
- If I have a bore gauge, the bore tells me instantly. Without one, the casting number is the answer.
The myth that valve cover style identifies displacement is false. Any small block accepts any small block valve cover, so a 305 with finned aluminum covers looks identical to a 350.
Step 5: Decode the VIN derivative (later engines)
Starting in the late 1960s, GM also stamped a partial VIN on the front deck pad below the suffix code. The partial VIN starts with the model year digit and includes the last six digits of the donor vehicle VIN. For numbers matching cars this stamping must match the car VIN tag exactly.
Common traps
- A swap meet seller writes 350 on a block based on the casting number alone. The casting may have started as a 350, but a previous owner could have bored it to 0.060 over (4.060) or even sleeved it back down. Always verify with a bore gauge before buying machine work.
- Mexican Goodwrench blocks (10066034) are common in salvage yards. They are excellent castings but were sold as replacement blocks, so they have no suffix code. The lack of a suffix is normal.
- 1986 and newer blocks use a one piece rear main seal. Earlier blocks use a two piece seal. The crank from a one piece block does not interchange with a two piece block without modifications.
The takeaway
To identify a small block Chevy, I read the casting number on the rear bellhousing flange for the raw block ID, then the suffix code on the front deck pad for the original application. Together they tell me displacement, year, main cap count, original horsepower, and intended vehicle. Cross reference both against a published GM decode guide and I will never order the wrong pistons, cam, or intake for a project engine.
