Squarebody Chevy Engine Guide (1973–1987)


Your complete guide to factory engine options for Chevrolet and GMC C/K Series trucks

From 1973 to 1987, Chevrolet and GMC square-body trucks came with a wide variety of engine options—ranging from reliable inline-sixes to torque-heavy big blocks. Whether you're restoring a factory setup or planning a swap, understanding the original engine lineup is the first step.


🛠️ Engine Options by Year


Inline-Six Engines

🔧 250 CID (4.1L) Inline-Six

  • Years Available: 1973–1984
  • Pros: Simple, reliable, great low-end torque, excellent fuel economy for the era
  • Cons: Limited power output, not ideal for heavy hauling or modern highway speeds
  • Common Issues: Worn timing gears, oil leaks from rear main seal, valve cover gasket seepage

🔧 292 CID (4.8L) Inline-Six

  • Years Available: 1973–1989 (mainly in heavier-duty models)
  • Pros: Stronger bottom end than the 250, torque-friendly for work trucks
  • Cons: Heavier and less fuel efficient than the 250
  • Common Issues: Cracked exhaust manifolds, valve train wear


Small Block V8 Engines

🔧 305 CID (5.0L) V8

  • Years Available: 1976–1987
  • Pros: Good economy for a V8, widely available, cheap to maintain
  • Cons: Lacks power compared to other small blocks, not ideal for towing
  • Common Issues: Flat cam lobes, carbon buildup in heads, oil consumption

🔧 307 CID (5.0L) V8

  • Years Available: 1973 only (phased out early)
  • Pros: Early small block heritage, decent low-end torque
  • Cons: Outdated by the mid-’70s, few performance parts specific to this engine
  • Common Issues: Oil leaks, worn valve guides

🔧 350 CID (5.7L) V8

  • Years Available: 1973–1987
  • Pros: Most popular and versatile engine, strong aftermarket support, reliable and powerful
  • Cons: Base versions were detuned in later years for emissions, varied in power across trims
  • Common Issues: Oil leaks, weak timing chain, worn valve seals


Big Block V8 Engines

🔧 366 CID (6.0L) Big Block V8

  • Years Available: Medium/heavy-duty trucks only
  • Pros: Built for hauling, reliable under load
  • Cons: Not commonly used in light-duty squarebodies
  • Common Issues: Valve seat wear, fuel delivery problems

🔧 396/402 CID (6.5L) Big Block V8

  • Years Available: 1973–1974 (402 was a continuation of the 396 with a new badge)
  • Pros: Excellent torque, classic muscle-era engine
  • Cons: Heavy, not fuel-efficient
  • Common Issues: Cooling issues, cam wear

🔧 454 CID (7.4L) Big Block V8

  • Years Available: 1973–1987 (mostly in 3/4 and 1-ton models)
  • Pros: Massive torque, excellent for towing, strong aftermarket support
  • Cons: Poor fuel economy, heavy, pricey to rebuild
  • Common Issues: Cracked heads (early castings), poor emissions components in later years


Diesel Options

🔧 5.7L Oldsmobile Diesel V8

  • Years Available: 1978–1981
  • Pros: Marketed for fuel economy during gas crises
  • Cons: Infamously unreliable, poor design, and fragile internals
  • Common Issues: Cracked blocks, failed head gaskets, weak head bolts

🔧 6.2L Detroit Diesel V8

  • Years Available: 1982–1987
  • Pros: Solid fuel economy, good torque for light towing, popular in military use
  • Cons: No turbo, slow acceleration, noisy
  • Common Issues: Cracked heads, starter woes, glow plug system failures


🔍 How to Identify Your Engine

Here’s how to figure out which engine your squarebody has:

1. Check the VIN (Vehicle Identification Number)

  • The 5th character of the VIN (for 1980–1987) indicates the engine type
  • Example: “H” = 305 V8, “M” = 350 V8, “N” = 454 V8
  • For 1973–1979, consult the build sheet or RPO code list under the hood or glove box

2. Look for the Engine Code Stamp

  • Found on a machined pad at the front passenger side of the block (below the head)
  • Example format: V0103TBC
  • V0103 = Flint, MI plant, Jan 3rd
  • TBC = Suffix that tells the engine spec/year (match with GM reference guide)

3. Casting Numbers

  • Found on the rear of the engine block (driver side) or under the distributor
  • These numbers tell you the block’s displacement and production year


🧰 Tips for Restorers and Swappers

  • Originality matters for collectors—matching numbers add value
  • The 350 is your go-to for balance between power, reliability, and parts support
  • The 6.2 diesel is a love/hate platform—avoid unless you really want diesel-specific traits
  • The 454 is the king of torque, perfect for tow rigs or heavy builds


1. VIN Number (Post-1980 Models)

  - 5th digit tells you the engine.

  - Example: "H" = 305 V8, "M" = 350 V8, "N" = 454 V8


2. Engine Code Stamp

  - Located on passenger side front pad on the engine block.

  - Example: V0103TBC

   - V = Flint, MI

   - 0103 = January 3rd

   - TBC = Engine suffix code


3. Casting Numbers

  - Found at rear driver side of block or under distributor.

  - Tells displacement and production year.



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1973–1987 Squarebody Chevy/GMC VIN Engine Decoder Guide

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VIN (Vehicle Identification Number) Breakdown

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- 1973–1980: 13-character VINs

- 1981–1987: 17-character VINs (standardized format)


For 1981–1987 trucks, the **5th character** of the VIN identifies the engine.


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5th VIN Digit (1981–1987) — Engine Code Reference

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  Code        Engine Type                     Displacement   Years Used 

|------|--------------------------------|--------------|-------------|

  H        V8 - 305 CID (LG4)                           5.0L           1981–1987   

  K       V8 - 350 CID (L83 / LS9)                  5.7L           1981–1984   

  M       V8 - 350 CID (L05)                            5.7L           1985–1987   

  N       V8 - 454 CID (L19)                             7.4L            1981–1987   

  J        I6 - 250 CID (L25)                               4.1L            1981–1984   

  L        I6 - 292 CID (L28)                              4.8L            1981–1987   

  C        V8 Diesel - 5.7L (LF9)                       5.7L            1978–1981* 

  F        V8 Diesel - 6.2L (J-series)                6.2L            1982–1987   


*Note: 5.7L LF9 Diesel was phased out around 1981. VIN codes for pre-1981 engines were not standardized.


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Engine Identification Tips for 1973–1980 VINs

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- VINs from 1973–1980 do **not** include engine data in a standard position.

- To identify the engine:

 1. Check the **emission sticker** under the hood.

 2. Look for **RPO (Regular Production Option) codes** inside the glove box.

 3. Check **engine casting numbers** and **engine stamp codes** on the block.


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Where to Find VIN and Engine Codes

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- VIN: Stamped on driver’s side dash near windshield.

- Engine Code: Passenger side front pad of block (below cylinder head).

- Casting Numbers: Rear driver side of engine block or under distributor.


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Inline-Six Engines

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250 CID (4.1L) — 1973-1984

- Pros: Simple and reliable. Good torque for daily use.

- Cons: Low power output. Not ideal for towing or long-haul use.

- Common Problems: Timing gear wear, rear main seal leaks, valve cover seepage.


292 CID (4.8L) — 1973-1989

- Pros: Great torque. Stronger than the 250. Durable.

- Cons: Heavier and uses more fuel than the 250.

- Common Problems: Cracked manifolds, valve wear.


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Small Block V8 Engines

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305 CID (5.0L) — 1976-1987

- Pros: Good fuel economy for a V8. Easy to find and maintain.

- Cons: Lower power. Not ideal for performance or towing.

- Common Problems: Flat cam lobes, oil consumption, carbon buildup.


307 CID (5.0L) — 1973 only

- Pros: Classic small block torque and feel.

- Cons: Rare, outdated, and less parts availability.

- Common Problems: Oil leaks, worn valve guides.


350 CID (5.7L) — 1973-1987

- Pros: Most popular and versatile engine. Huge aftermarket support.

- Cons: Power was detuned in later years for emissions.

- Common Problems: Weak timing chain, oil leaks, valve seal wear.


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Big Block V8 Engines

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366 CID (6.0L) — Medium/Heavy duty only

- Pros: Built for work trucks. Tough under load.

- Cons: Rare in light-duty trucks.

- Common Problems: Valve seat wear, fuel delivery issues.


396/402 CID (6.5L) — 1973-1974

- Pros: Great torque. Classic muscle-era engine.

- Cons: Heavy and poor MPG.

- Common Problems: Overheating, cam wear.


454 CID (7.4L) — 1973-1987

- Pros: Incredible torque. Great for towing and big builds.

- Cons: Fuel hungry and heavy.

- Common Problems: Cracked heads, weak emissions parts.


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Diesel Engines

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5.7L Olds Diesel — 1978-1981

- Pros: Designed for fuel economy.

- Cons: Poor reputation. Fragile internals.

- Common Problems: Cracked blocks, failed head gaskets, weak head bolts.


6.2L Detroit Diesel — 1982-1987

- Pros: Good fuel economy and torque. Long lifespan when cared for.

- Cons: No turbo. Loud and slow.

- Common Problems: Glow plug issues, starter failures, cracked heads.


🔚 Final Thoughts: Squarebody Engine Guide Wrap-Up

Whether you're restoring a survivor, daily-driving a classic, or building a weekend trail rig, knowing the ins and outs of your Squarebody’s factory engine is a must. From VIN decoding and engine ID tips to the strengths and weaknesses of every powerplant offered from 1973 to 1987, this guide was built to give you the most complete engine reference available anywhere for Chevy and GMC Squarebody trucks.

These trucks were made to last—and with the right knowledge, you can keep them running stronger than ever.

Got questions about what engine you're working with or planning a swap? Reach out or explore more of our tech articles here at Blazin’ Biddles Off-Road, where we live and breathe Squarebody Chevys.