Chevy Small Block Casting Numbers: The Complete Decoder Guide

Staring at a mystery engine with a string of numbers stamped into the iron? Those Chevy small block casting numbers tell you everything — displacement, foundry origin, production date, and whether it’s a numbers-matching gem or a mongrel swap. This guide breaks it all down so you can decode any small block with confidence. Stick around — the details at the end could save you thousands.

What Are Chevy Small Block Casting Numbers?

Chevy small block casting numbers are identifiers cast directly into the iron at the foundry. They appear in raised relief on the block’s exterior. Think of them as the engine’s birth certificate — permanently embedded at the moment of creation.

Every small block carries three separate data points you need to decode properly:

• The block casting number — reveals the base architecture and intended displacement
• The casting date code — pinpoints the exact day the iron was poured
• The assembly stamp (suffix code) — confirms the finished engine’s exact specs and vehicle application

Miss any one of these three, and you’re only telling part of the story.

Where to Find Casting Numbers on a Small Block Chevy

Location matters. Here’s exactly where to look:

• Main casting number: Rear of the block, driver’s side, on the upper bellhousing flange
• Casting date code: Rear passenger side, on the bellhousing flange opposite the casting number
• Assembly stamp pad: Front passenger side, just ahead of the cylinder head deck surface

The casting number sits in raised metal — it’s part of the block itself. The assembly stamp is machined into a flat pad, so those numbers are recessed rather than raised.

How to Read a Casting Date Code

The casting date code uses a simple alphanumeric format. Here’s how it breaks down:

One important quirk: Chevrolet used the letter “I” for September — unlike carburetor manufacturers who skipped “I” to avoid confusion with the number one.

Another quirk: A single-digit year code like “9” could mean 1959, 1969, or 1979. You must cross-reference it against the casting number’s known production years to nail the correct decade.

The Tonawanda foundry often used a two-digit year code — so “66” means 1966, no guesswork needed.

If you spot an “M” or “X” where the month code should be, that’s a prototype or test casting. The “M” came from Saginaw, and the “X” from Tonawanda. Standard date rules don’t apply to these.

Decoding the Assembly Stamp Pad

The stamp pad is where the engine’s full identity lives. It gets stamped after full assembly, and it tells you:

Assembly plant prefix letters:

• F — Flint, Michigan (1955–1966)
• V — Flint V8 Plant (1967 onward)
• T — Tonawanda, New York
• K — McKinnon Industries, Ontario, Canada

After the plant code come four digits representing the assembly date (month and day). Then the suffix code — two or three characters that identify the displacement, horsepower, transmission pairing, and exact vehicle platform.

Pro tip: Assembly plants never used the numeral “1” in their stamping dies. They used the capital letter “I” instead. So “T02I4” means Tonawanda, February 14th — not February 14th with a random number one.

The casting date must always precede the assembly date, typically by days to a few weeks. A gap up to six months is historically normal due to parts stockpiling.

The VIN Derivative: Proving “Numbers Matching”

Starting in 1968 for all passenger vehicles (earlier for Corvettes), a partial VIN was stamped alongside the assembly code on the front passenger pad.

The VIN derivative is nine characters: Chevrolet division code, model year digit, assembly plant letter, and the last six digits of the chassis VIN. When those six digits perfectly match your vehicle’s chassis tag, you’ve got a numbers-matching engine — and that status dramatically increases collector value.

On certain 1969 Z28 Camaros and high-performance Chevelles, the alternator bracket blocked the front pad on the assembly line. Workers stamped the VIN derivative near the oil filter housing on the lower rear passenger side instead.

Chevy Small Block Casting Numbers Reference Tables

265, 283, 302, 305, and 307 Cubic Inch Blocks

These early and economy-era blocks established the small block’s foundation. The legendary 302 was built by dropping a short 3.000-inch stroke 283 crankshaft into a 4.000-inch bore 327 block — purely to meet Trans-Am racing rules.

327 Cubic Inch High-Performance Blocks

The 327 ruled the mid-1960s performance scene, powering top-spec Corvettes and early muscle cars with a 4.000-inch bore and 3.250-inch stroke.

350 Cubic Inch Blocks

The 350 launched in the 1967 Camaro SS and quickly became the universal GM standard. It’s the most common small block you’ll encounter.

400 Cubic Inch Blocks

The 400 cubic inch small block pushed the architecture to its absolute limit. Engineers had to increase the bore to 4.125 inches, which forced a “siamesed” cylinder wall design with no water jacket between adjacent cylinders. That’s why 400 blocks require unique steam holes drilled into the deck and matching heads to prevent overheating.

Engineering note: The 3951511 four-bolt is coveted, but the massive bores and drilled main holes leave the webbing thin. For serious horsepower, many builders prefer a two-bolt 3951509 fitted with aftermarket splayed main caps — stronger inner webs, wider clamping angle.

Saginaw vs. Tonawanda: How to Tell Foundries Apart

Two foundries built most of these engines. You can tell them apart before you even read a casting number.

Saginaw (Flint) blocks have a square-head 1/8-inch NPT plug at the 11 o’clock position on the front face of the block, above the timing cover. This plug seals an oil gallery drilling channel. Tonawanda blocks don’t have this hole at all — they used a different machining process.

Tonawanda blocks often feature a prominent cast letter “T” somewhere on the component itself.

Both foundries also integrated a “casting clock” into their molds — a circular arrangement of ten raised dots representing a ten-hour shift. An arrow marks the shift start, and an offset dot indicates the specific pour hour. This let quality control engineers trace any defective batch to an exact shift and crew.

McKinnon Industries in Ontario used a “K” prefix on the assembly stamp. These blocks went into Canadian-market vehicles but also crossed the border when domestic plants ran hot. McKinnon even supplied small block Chevys to Studebaker for their 1965–1966 models after Studebaker closed its own engine plant.

Two-Bolt vs. Four-Bolt Mains: What Actually Matters

The two-bolt versus four-bolt debate is real, but it’s more nuanced than most people think.

Two-bolt mains use two vertical fasteners per cap. They’re strong, reliable, and perfectly adequate for street performance and standard horsepower levels.

Four-bolt mains add two extra fasteners on caps two, three, and four. The wider clamping footprint prevents “cap walk” — the destructive shifting of main caps under high cylinder pressures from forced induction or nitrous. These blocks were standard on heavy-duty trucks, high-compression Corvettes, and race applications.

The twist: A two-bolt block actually retains more iron in the main webbing because it lacks the extra drilled holes. A quality two-bolt block with an aftermarket main stud girdle often outperforms a factory four-bolt in ultimate strength — because the girdle ties all five caps together and distributes load across the entire assembly.

You can’t tell two-bolt from four-bolt by looking at the outside. Removing the oil pan and counting fasteners is the only definitive method. Suffix codes indicating high-performance applications (like a Z28 or high-output Corvette) are a strong clue toward four-bolt, but never assume.

The 010 and 020 High-Nickel Blocks

Certain casting numbers like 3970010 appear with additional markings stamped near the timing cover or rear bellhousing flange. These numbers decode as follows:

• 010 — 10% added nickel in the iron alloy
• 020 — 20% added nickel
• 010/020 combined — the top-shelf mix: 20% nickel and 10% tin

The nickel increases tensile strength and hardens cylinder walls. The tin refines the iron’s grain structure for better machinability and thermal stability. These blocks resist cylinder wall cracking under extreme heat and high-boost conditions.

Folklore says these were reserved for performance cars. Reality is different — high-nickel pours primarily went to heavy-duty commercial trucks and specialized racing blocks. Finding one in a muscle car is fortunate timing from shared foundry molds, not exclusive performance allocation.

Service Replacement “CE” Blocks

When a factory-warrantied engine failed during the muscle car era, dealers needed immediate replacements. GM produced dedicated service units stamped with the prefix “CE” — standing for Chevrolet Engine.

CE blocks intentionally lack vehicle-specific suffix codes since they were universal replacements. The format runs: CE + year digit + sequential production number. A stamping of “CE839012” means a 1968 Chevrolet Engine, the 39,012th unit in that plant’s sequence.

When a plant exhausted its numerical allotment, they appended the letter “A” and restarted. CE blocks never received a chassis VIN derivative — they were counter parts, not line-built units. That voids strict numbers-matching status, but a CE block is a historically authentic factory intervention that carries real provenance in restoration circles.

Engine Suffix Code Samples

Suffix codes are the final piece of the puzzle. Here’s a sample of what they reveal:

GM reused codes across different eras, so always establish the assembly date first before interpreting a suffix code. The same three letters can mean two completely different engines depending on the decade.

The Quick Verification Checklist

Before you commit to any small block purchase or restoration, run through this fast forensic check:

1. Find and record the casting number — confirm displacement and production era match
2. Decode the casting date — letter for month, two digits for day, single digit for year
3. Cross-reference date against casting number production years — confirms correct decade
4. Read the assembly stamp — plant code, assembly date, suffix code
5. Check the VIN derivative — six digits must match chassis tag for numbers-matching status
6. Look for 010/020 marks — above timing cover or at rear bellhousing flange
7. Check for the Saginaw oil gallery plug — 11 o’clock position on the front face

Run those seven steps, and no small block can hide its past from you.