Squarebody Chevy Transmission Guide (1973–1987)

Complete Factory Automatic and Manual Transmission Reference for Chevrolet and GMC C/K-Series Trucks

Between 1973 and 1987, Chevrolet and GMC Squarebody trucks were offered with a wide range of factory transmissions. From heavy-duty automatics like the TH400 to rugged manual gearboxes like the SM465, each transmission had its own strengths, weaknesses, and best-use scenarios.

Understanding your transmission is key whether you’re restoring a factory setup, planning an upgrade, or swapping driveline components. This guide breaks down every transmission option available in Squarebody Chevy and GMC trucks from 1973 through 1987, with identification tips, pros and cons, and common failure points.

Automatic Transmissions

TH350 (Turbo Hydramatic 350)

  • Years: 1973–1986
  • Type: 3-speed automatic
  • Pros: Lightweight, reliable, widely available, inexpensive to rebuild
  • Cons: No overdrive, not ideal for towing or highway fuel economy
  • Common Issues: Worn clutches, leaking seals, detent cable problems

TH400 (Turbo Hydramatic 400)

  • Years: 1973–1987
  • Type: 3-speed automatic
  • Pros: Extremely strong, excellent for towing and big block applications, long service life
  • Cons: Heavy, robs more engine power, no overdrive
  • Common Issues: Modulator failure, kickdown switch failure, fluid leaks

TH700R4 (later renamed 4L60)

  • Years: 1982–1987
  • Type: 4-speed automatic with overdrive
  • Pros: Overdrive improves highway MPG, wide gear ratio spread, lowers cruising RPM
  • Cons: Early models (1982–1986) prone to reliability issues until factory upgrades were made
  • Common Issues: Weak 3-4 clutch packs, TV cable misadjustment, torque converter failure

TH200C

  • Years: 1981–1984
  • Type: 3-speed automatic
  • Pros: Light-duty option with basic operation
  • Cons: Too weak for full-size truck use, rare today
  • Common Issues: Premature wear, slipping, limited rebuild support

TH200-4R

  • Years: 1984–1987 (rare in trucks)
  • Type: 4-speed automatic with overdrive
  • Pros: Overdrive for better fuel economy, compact size
  • Cons: Not common in Squarebody trucks, parts harder to source
  • Common Issues: Clutch wear, line pressure issues

Manual Transmissions

SM465

  • Years: 1973–1987
  • Type: 4-speed manual with “granny low” first gear
  • Pros: Nearly indestructible, perfect for off-road and towing, simple design
  • Cons: No overdrive, heavy, not smooth-shifting
  • Common Issues: Worn 2nd-gear synchros, sloppy shifters

SM420

  • Years: Installed until about 1977 in some early trucks
  • Type: 4-speed manual
  • Pros: Legendary strength, proven design
  • Cons: Outdated by the mid-’70s, fewer parts available
  • Common Issues: Worn gears, hard-to-find replacement parts

NP435

  • Years: 1973–1979 (mostly GMC and heavy-duty trucks)
  • Type: 4-speed manual
  • Pros: Very strong, comparable to the SM465, heavy-duty gearing
  • Cons: Rare in Chevy trucks, limited aftermarket
  • Common Issues: Similar wear points as SM465

MY6 (New Process A833 Overdrive)

  • Years: 1981–1987
  • Type: 4-speed manual with overdrive 4th gear
  • Pros: Offers OD in a manual, better fuel economy, lighter than SM465
  • Cons: Rare, not as durable for heavy towing
  • Common Issues: Synchro wear, limited replacement parts

Saginaw 3-Speed

  • Years: 1973–1984
  • Type: 3-speed manual (column shift in base models)
  • Pros: Simple, inexpensive, easy to repair
  • Cons: Weak for heavy loads, no overdrive, not performance-oriented
  • Common Issues: Worn linkage, sloppy shift feel, grinding gears

Borg Warner T-10

  • Years: Rare in performance-oriented trucks only
  • Type: 4-speed manual
  • Pros: Performance-focused gearbox, good for muscle-truck builds
  • Cons: Rare in Squarebodies, limited applications
  • Common Issues: Gear wear, parts harder to locate

How to Identify Your Transmission

  1. VIN & RPO Codes
  • VIN does not list transmissions directly.
  • Check glovebox RPO (Regular Production Option) codes or service records.
  1. Transmission Pan Shape (Automatics)
  • TH350: Square pan with one corner cut off
  • TH400: Larger, irregular pan with hump on side
  • TH700R4: Rectangular pan with stepped corners
  1. Casting Numbers & Stamps
  • Found on the driver’s side bellhousing or tailshaft.
  • Manuals often stamped with GM part numbers.
  1. Shifter Location (Manuals)
  • SM465: Tall top-mounted tower shifter
  • Saginaw 3-speed: Column-mounted shifter
  • MY6 (A833): Side-shifter design similar to car applications
  1. Bellhousing Bolt Pattern
  • Bolt count and location help narrow identification.

Closing Thoughts

The 1973–1987 Squarebody Chevy and GMC trucks were offered with one of the most diverse transmission lineups in GM history. From the bulletproof SM465 manual to the highway-friendly 700R4 automatic, each option served a specific purpose depending on the truck’s role.

For restorers, knowing the factory transmission ensures authenticity. For builders, understanding strengths and weaknesses helps in choosing the right transmission for towing, off-roading, or daily driving.

At Blazin’ Biddles Off-Road, we specialize in drivetrain knowledge, parts, and packages built specifically for Squarebody Chevy/GMC owners. Whether you’re keeping it stock or planning an upgrade, our guides and products are here to help you get the most out of your truck.