Identifying a small block Chevy by its casting numbers

Everything here works with the engine in the car or on a stand. You need a flashlight, an inspection mirror, something to degrease with, and a notepad. Write numbers down as you find them instead of trusting your memory in the driveway.

Clean the surfaces before you read anything

Cast characters fill with paint, road film, and rust scale. Work on a cold engine only. Brake cleaner and engine degreaser are flammable and their vapors are harmful, so keep them away from hot exhaust, sparks, and any open flame, open the garage door or work outside, and wear eye protection and gloves. With the engine cold and the area ventilated, degrease the surface, then work it with a brass brush and brake cleaner until the digits stand up clearly.

Do not put a grinder or a hard wire wheel anywhere near the stamped pad on the deck. Those characters are only a few thousandths deep, and abrasive cleaning erases them. It also leaves marks that look like tampering to anybody who values the car later. Hand work only in that spot.

Once a number is legible, photograph it with light raking across the surface from one side. A rubbing taken with thin paper and the side of a soft pencil often reads better than a photo.

The block casting number at the bell housing flange

The block casting number sits at the rear of the block, on the bell housing flange, driver side, up near the top of the flange. It is cast into the iron in raised digits, seven of them on earlier blocks and eight on later ones.

With a transmission bolted up this is an awkward spot. Reach it from above with a mirror, or from underneath with the car raised. If you go underneath, park on level concrete, chock the wheels, and support the car on rated jack stands. Never get under a car held up by a jack alone.

Clean it, read it twice, and copy it exactly. A greasy 6 reads as an 8, a 3 reads as an 8, and a single wrong digit sends you to the wrong casting entirely.

Cross reference what you find against a published casting reference rather than one forum post.

Reading the casting date code

The casting date code is a short code near the casting number on the same rear flange area. The exact spot moves between castings, so clean a wide area and look around before you decide it is missing.

The format is a letter, then one or two digits, then one more digit. The letter is the month, with A for January running straight through to L for December. The middle digits are the day of the month. The last digit is the last digit of the year.

Because the year is a single digit, the date code alone cannot separate one decade from the next. The casting number does that for you.

The date is when the iron was poured. Assembly came days or weeks later, and the car was built after that. An original block carries a date shortly before the car's build date. A date falling after the car was built proves the block is not original.

The assembly stamp on the front pad

There is a machined pad on the deck surface at the front of the passenger side cylinder head. Original engines carry a stamped code there, not a cast one.

You are usually looking at two groups of characters. One is the engine assembly code: a letter for the engine plant, a numeric month and day, and a suffix of two or three letters. That suffix is the useful part, because it identifies the application, meaning the car line, model year, horsepower rating, and transmission the engine was built for.

The other group is a partial VIN matching the car the engine was installed in. That is what people mean by numbers matching on a Chevy.

Suffix codes were reused across years, so decode them against a reference for the specific model year rather than from memory.

Why that pad is usually blank

Most small blocks you meet have a blank pad, and that is normal rather than suspicious.

Decking is the usual reason. When a rebuilder mills the deck flat, the cut goes deeper than the stamp and the stamp goes with it. That is standard practice on a rebuild, not a cover up. Service replacement short blocks also left the factory without a car VIN and were marked in their own way. Add corrosion, thick paint, and the occasional owner who took a grinder to it.

Restamped pads do exist. Uneven character spacing, a font that does not match the surrounding work, a pad sitting noticeably below the deck, or fresh machining around it are all reasons to slow down. If real money depends on the answer, pay somebody who authenticates these engines for a living.

Cylinder head casting numbers

Head casting numbers are under the valve covers, cast into the top surface of the head. Look along the center rail between the rocker studs and at the ends of the head. Some castings carry the number on the end face instead.

Pull the covers to read them, and have gaskets on hand first so the car is not stuck apart waiting on a parts order.

Heads carry their own date codes in the same letter, day, year format. On an untouched engine the head dates land close to the block date and usually just after it. Two heads with dates years apart tell you somebody has already been in there.

A head casting hints at chamber size and valve size. Those are hints. Measure the chamber and the valves if the answer matters.

What a casting number cannot tell you

A casting number identifies a casting. That is all it does, and the limits are worth being blunt about.

It does not give you displacement. The bore is machined after the iron is poured, and one casting served several displacements. Stroke comes from the crankshaft, which is a separate part with its own number, so the block has no way of knowing what is turning inside it.

It does not give you the model year of the car. Castings ran for years and went into cars, trucks, marine engines, and over the counter service parts.

It does not reliably tell you two bolt or four bolt mains, and it says nothing about the current bore. Both of those need looking at in person.

Finding the actual displacement

To learn displacement, measure. Read the bore with a caliper across the top of a cylinder, or better with a bore gauge. Get the stroke from the crank, either by measuring piston travel in one cylinder or by reading the casting number on a counterweight.

Then run the math. Displacement equals pi divided by four, times bore squared, times stroke, times eight cylinders. Work in inches and the answer comes out in cubic inches. Compare it against the published displacements for the family and take the nearest one.

A block that has been rebuilt before will measure larger than the factory bore, which tells you roughly what oversize it is already at. Treat that as a reading, not an order sheet. A worn or damaged bore has to be cleaned up before it is final, so buy pistons and rings to the finished size your machine shop gives you after it bores the block, never to the year and never to a worn measurement.

Assume it is a rebuild and buy parts that way

Treat any engine as rebuilt until the evidence says otherwise. Heads get swapped, blocks get bored, cranks get replaced, and complete crate engines get dropped in. The numbers on the iron describe the parts in front of you, not the car's history.

Buy parts the same way. Check the rear main seal arrangement at the rear main cap before ordering a gasket set, because early and late blocks use different designs. Look at whether the valve covers bolt around the perimeter or through the center before ordering covers or gaskets. Confirm bolt patterns and port shapes by looking rather than assuming.

Write every number you find into the car's file with a photo attached. You will want them again at the parts counter.

Common questions

Can I get the displacement from the block casting number?

No. The casting number identifies the raw casting. Bore size is machined afterward, and one casting served several displacements. Stroke comes from the crankshaft, which is a separate part with its own number. Measure the bore and the stroke if you need the real answer.

The pad in front of the passenger side head is blank. Is the engine a fake?

Usually not. Decking a block during a rebuild cuts deeper than the stamp and takes it off, and that is routine work. Service replacement blocks also left the factory without a car VIN. A blank pad means that piece of evidence is gone, not that something is wrong.

Does the casting date tell me the model year of the car?

No. It tells you when the iron was poured. On an original engine the casting date falls shortly before the car's build date, so the two are related but not the same. A casting date later than the car's build date proves the block is not original.

Do I have to pull the heads to read the head casting numbers?

No, just the valve covers. The numbers are cast into the top surface of the head, usually in the center rail or at the ends. Have valve cover gaskets on hand before you take the covers off.