How to Decode the GM Engine Block

Decoding the engine block on a GM vehicle can provide a wealth of information about the engine. With the right codes, you can identify when and where the engine was manufactured, the size, horsepower, transmission applications, and more. This guide will walk you through the entire process of decoding a GM engine block.

Locating the Engine Block Code

The first step is finding the engine block code, which is a series of 7-8 digits stamped directly onto the engine block. The location of this code depends on the type of engine:

  • Small Block Chevy: Machined pad in front of the passenger side cylinder head[1].
  • Big Block V8: Stamped above the timing chain cover[3].
  • 6 Cylinder: Behind the distributor on the passenger side of the block[3].

Before decoding the engine block, wipe away any grease or dirt so you can clearly read each digit in the code. Make sure you can see the entire code – some digits may be partially hidden by brackets or other engine components.

Decoding the Engine ID Code

GM engine codes follow a standard format:

The first digit is a letter indicating the manufacturing plant:

  • F = Flint, Michigan
  • H = GM Hydra-Matic transmission plant
  • K = St. Catherines, Ontario
  • M = Toluca, Mexico
  • S = Saginaw, Michigan
  • T = Tonawanda, New York
  • W = Willow Run, Michigan
  • Z = Fremont, California

The second digit specifies the engine division:

  • 1 = Chevrolet Passenger Car
  • 2 = Chevrolet Truck
  • 3 = Chevrolet Truck
  • 4 = Chevrolet Passenger Car
  • 5 = Chevrolet Passenger Car
  • 6 = Chevrolet Light Truck
  • 7 = GM of Canada
  • 8 = Oldsmobile
  • 9 = Pontiac

The third digit indicates the assembly plant:

  • 0 to 9 = Various GM assembly plants

The fourth digit is the model year:

  • 7 = 1967
  • 8 = 1968
  • 9 = 1969
  • 0 = 1970
  • …and so on. Add 1960 to this digit to get the model year.

The fifth digit specifies the engine series:

  • Z = 454 Big Block
  • R = 396 Big Block
  • K = 327 Small Block
  • T = 400 Small Block
  • S = 350 Small Block
  • P = 302 Small Block
  • etc.

The sixth digit is the engine suffix code, which further identifies features of that particular engine, such as horsepower, transmission, carburetion, emissions equipment and more. These vary significantly depending on the engine series.

The seventh and eighth digits are the engine sequence number indicating the production sequence of that engine.

So for example, an engine stamped “T0510SJA” indicates:

T: Tonawanda, NY assembly plant

05: Chevrolet carline

1: Chevrolet Detroit assembly plant

0: 1970 model year

S: 350 cubic inch Small Block

J: 350hp Turbo-Fire V8

A: Sequential production number

Therefore, this decodes to a 1970 Chevrolet 350 cubic inch, 350hp Turbo-Fire V8 produced at Tonawanda.

Using Casting Numbers

In addition to the engine ID code, many GM blocks also have a casting number stamped elsewhere on the block. This is a 6-7 digit code that gives more specifics on the block’s engineering characteristics such as material used, shape, size and more.

Casting numbers are helpful for identifying bare engine blocks without accompanying parts. Some common small block casting numbers include:

  • 3970010 – Small Block V8, 262ci
  • 3998993 – Small Block V8, 265ci
  • 3932388 – Small Block V8, 307ci
  • 14010207 – Small Block V8, 327ci
  • 3858174 – Small Block V8, 350ci

Casting numbers can be used along with visual cues to identify bare engine blocks. Look for unique physical features of the block:

  • Bellhousing bolt pattern
  • Engine mount locations
  • Oil pan shape
  • Lifter valley shape
  • Cylinder head bolt pattern

Combining the casting number, visual inspection and any partial ID codes can help narrow down the identity of a bare GM block.

Verifying Engine Size

Once you’ve decoded the ID stamping and casting numbers, it’s a good idea to physically verify the displacement to confirm the decoded size is correct.

Use a bore gauge to measure cylinder bore diameter, and a telescoping gauge to measure crankshaft stroke. Then calculate:

Displacement = Bore x Bore x Stroke x 0.7854 x Number of Cylinders

For example, a typical 350 Small Block has a 4.00″ bore and a 3.48″ stroke.

4.00 x 4.00 x 3.48 x 0.7854 x 8 cylinders = 350 cubic inches displacement.

Measuring the bore and stroke yourself helps verify that the block has not been modified from its original size.

Additional Engine Stampings

Some GM blocks may have additional stampings beyond the engine ID code and casting number:

  • Partial VIN Number: On some engines, the last 6 digits of the vehicle VIN are stamped to identify the vehicle the engine belonged to. This is often stamped on the front passenger side of the block.
  • Engine Revision Number: This 4 digit code indicates any revisions made during the model year. Stamped on the distributor pad.
  • CE Number: On later model fuel-injected engines, the 8 digit CE number identifies the calibration used on that engine. Stamped on the front of the block.
  • RPO Codes: These 3 digit Regular Production Option codes identify special options for that engine, such as HD cooling or high performance packages. Typically found on a machined pad near the distributor.

These additional stampings can provide further detail on the engine’s origins and options. Cross-referencing them with build records can reveal exactly how a particular GM engine left the factory.

Small Block Chevy Engine Families

GM’s Small Block Chevy engine was produced continuously from 1955-2003, with five distinct generations, or families. Identifying the family helps determine compatibility with related engine parts. The major Small Block families are:

  • Gen I (1955-1992) – The original Small Block with a traditional front sump oil pan. Produced in sizes from 265-400 cubic inches.
  • Gen II (1992-1997) – “LT” engines featured updated electronics, fuel injection, and “Opti-Spark” distributor. All Gen II engines are 350 cubes or less.
  • Gen III (1997-2007) – Redesigned with a front sump and rear sump variants. “LS” engines featured deep-skirt blocks, distributorless ignition, and coil-near-plug ignition.
  • Gen IV (2005-2015) – Continuation of the Gen III LS design with greater strength and refinement.
  • Gen V (2014-present) – Current generation Small Block, marketed as “LT” and “EcoTec3” engines. Direct injection, variable valve timing, and active fuel management.

The engine family must be identified to ensure matching all related components like intake manifolds, engine mounts, accessory drives, oil pans, and more. Mixing parts between generations is generally not recommended.

Big Block Chevy Engine Families

GM Big Block engines followed a similar generational evolution as the Small Block:

  • Gen I (1965–1990) – Original Big Block family included 396, 427, and 454 cubic inch engines. Common nicknames are Mark I, Mark II, Mark III, and Mark IV.
  • Gen II (1991–1995) – Also called the “Big Block Chevy Vortec.” Featured updated cylinder heads, fuel injection, and improved block strength. Displacements of 454 and 502 cubic inches.
  • Gen III (1999–2009) – Last production Big Blocks. All-aluminum 502 engines or iron block 8.1L Vortec truck engines.

Parts are not generally interchangeable between these three families. Proper identification is critical to engine building with the correct components.

Researching GM Engine Specs

With the engine properly decoded, you can research specifications and parts using GM documentation and enthusiast resources:

  • Factory Service Manuals – Contain extensive engine specifications for each engine code, model year, and application. Available as printed manuals or digital subscriptions.
  • Parts Catalogs – List replacement parts for each engine ID. Useful for identifying compatible components for your engine.
  • Online Forums – Enthusiast discussion forums have detailed engine data. Searching by engine code provides build advice from experienced members.
  • Chevy Engine Books – Printed guides like “How to Rebuild Big-Block Chevy Engines” and “How to Rebuild Small-Block Chevy” contain comprehensive engine data.
  • Chevy Websites – Many websites compile specs for various GM engines. These provide quick references for engine details.

Thorough research using GM reference materials ensures you have the right specifications for your particular engine code and application. This helps avoid potentially incorrect or incompatible parts substitutions.

Decoding GM Transmission Codes

Identifying the transmission is equally important, as the engine and transmission combinations are specially calibrated. GM transmission codes provide details on the model, year, plant, and more.

GM automatic transmission codes are stamped on the ID tag on the transmission case. A typical example is “MYA 715,” which breaks down as:

  • M = Muncie 4-speed manual transmission
  • Y = Year (M=1965)
  • A = Assembly plant
  • 715 = Specific transmission model

So this decodes to a Muncie 4-speed manual transmission, built in 1965 at the Muncie, Indiana plant, part number 715.

Codes for Turbo-Hydramatic (THM) automatic transmissions follow a similar format:

  • THM400 – RPO code for popular 3-speed automatic
  • MD8 – Assembly plant code
  • 15N – Production code
  • 1984 – Model year

GM has used complex naming and coding schemes for their transmissions over the decades. Identifying the transmission is essential to finding the right replacement parts. As with engines, factory service manuals provide detailed specifications for each transmission code.

Ordering Correct Replacement Parts

With the engine and transmission fully decoded, you can accurately identify replacement parts:

  • Crate engines – Complete replacement engines are available from GM and aftermarket suppliers, avoiding a full rebuild. Match by engine code and size.
  • Rotating assemblies – Cranks, pistons, rods and rings can be ordered as a matched set for your engine’s bore, stroke, and compression.
  • Engine blocks – For engine rebuilds, matching the block casting number ensures the correct deck height, bore spacing, bellhousing pattern, etc.
  • Cylinder heads – Heads have specific valve sizes, CC volumes, intake manifold bolts, and performance characteristics for each engine.
  • Intake manifolds – Match by engine family and performance level. Pay attention to port sizes and carburetor/injector configurations.
  • Oil pans – Pans are often engine-family specific. Front sump, rear sump, oil pump pickup location, and oil capacity vary.

Carefully identifying your engine and transmission codes takes the guesswork out of ordering compatible parts. Always double-check that replacement parts precisely match your engine’s specifications. This saves time, money and avoids potentially unusable parts.

Performing a Partial Decode

If engine stampings are missing or unreadable, you can still perform a partial decode using clues on the block:

  • Measure bore and stroke to determine displacement. This narrows down the engine family.
  • Note the block material – iron or aluminum. Aluminum blocks are generally Gen III/IV Small Blocks or Big Blocks.
  • Count the head bolts per cylinder – Gen I/II Small Blocks have 12 bolts, Gen III/IV have 6 bolts.
  • Check lifter bore diameter – 0.842″ is Small Block, 1.00″ is Big Block.
  • Look for engine mount and bellhousing bolt patterns. These indicate Small Block, Big Block, or LS families.
  • Check for one- or two-piece rear main seals. Early Gen I Small Blocks had two-piece seals.
  • Look for side motor mount bosses – Present on Gen III/IV engines.

Careful visual inspection and measurement can help identify GM blocks even with partial or missing codes. This process takes patience but can help determine the general engine family.

Consulting an Expert

For GM engines that are very difficult to decode, consulting an experienced engine builder or specialist may be required. They can inspect:

  • Unusual block castings and features that indicate developmental engines or low production models.
  • Irregularities in casting details that don’t match published specifications.
  • Anomalies in production that point to short-lived manufacturing processes.
  • Markings left by the engineering department on prototype engines.
  • Specialized features for marine, industrial or non-automotive applications.

Citations: [1] [2] [3] [4] [5] [6]

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