Reading GM casting numbers and date codes
How to work out what block, head or manifold you actually have — and whether it is the one that left the factory in your car.
Before you buy parts for an engine, it is worth knowing what the engine is. A surprising number of classic cars are running something other than what the title implies, and the casting numbers will tell you in about ten minutes.
01
Block casting number — on the bell housing flange at the back of the block, on the driver's side, facing up. You will need a torch and a rag. It is a seven or eight digit number. Head casting number — under the valve cover, on the top rail between the springs, or on the end of the head. Intake manifold — on the top surface, usually under the carburettor or beside the thermostat housing. Exhaust manifold — raised on the outer face, often obscured by fifty years of scale. A wire brush finds it.
02
The casting number identifies the *part design*, not the individual part. It tells you what the piece is: a 350 four-bolt main block, a 462 head with 1.94 inch valves, a particular intake pattern. It does not tell you what year your specific one was cast. That is the date code, and it is a separate marking.
03
Next to the casting number there is usually a shorter code — a letter and two or three digits. The letter is the month (A is January through L is December, and I is skipped because it reads as a one). The digits are the day and the last digit of the year. So `C 15 8` is 15 March, in a year ending in 8. Which year ending in 8? The casting number narrows it. A block design that was only produced from 1969 to 1973 with a date code ending in 8 does not exist; one ending in 9 or 0 does. Between the two markings you can normally pin it down exactly.
04
For buying parts, it tells you what you have. A 350 and a 400 small block look nearly identical from three feet away and take different gaskets, different harmonic balancers and different bellhousings. Guessing based on what the previous owner said is how you end up with a gasket set you cannot return because you opened the box. For originality, the date code is the thing. A numbers-matching car has an engine cast a sensible interval before the car was assembled — typically a few weeks to a couple of months. A block cast eighteen months after the car was built is a replacement, whatever anybody claims. That is not necessarily bad. Most cars of this age have had an engine out at some point, and a well-built replacement is often a better engine than a tired original. But it changes the value substantially, and you should know before you pay, not after.
05
A casting number is not a part number. They are different numbering systems, and searching a casting number in a parts catalogue that indexes part numbers finds nothing at all — which people reliably interpret as "this part is unavailable" rather than "I searched the wrong number." We index both. If you have a number and are not certain which kind it is, put it in the search box and see what comes back.
06
Fifty years of scale, paint and undercoating hide a lot. A wire brush and some solvent usually finds it. If the casting is genuinely too corroded, physical measurements will get you there — bore, stroke, deck height, head bolt count. Or send us photographs with something for scale. We look at these all day and can usually identify a casting from the shape alone.
Square-body buyer's guide: what rust actually costs
Six failure points that decide whether a 1973–1987 C10 is a project or a money pit, and how to check each one in the ten minutes you get before committing.
Buyer's guidesReproduction, NOS or used: choosing parts for a driver versus a show car
The three ways to buy a classic part, what each actually costs you, and why the right answer changes completely depending on what you are building.
Buyer's guidesEarly Bronco buyer's guide: where they rot and what it costs
Values have climbed far enough that a rusty Bronco now costs real money. Here is where to look before you agree a price, and what the bad news is worth.