🔧 Squarebody Chevy Transmission Guide (1973–1987)
Complete breakdown of every factory automatic and manual transmission option in Chevy/GMC C/K-Series trucks.
🔧 Introduction: Understanding Squarebody Transmissions (1973–1987)
From daily drivers to weekend crawlers and trail rigs, Squarebody Chevy and GMC trucks are some of the most versatile platforms ever made. But behind every dependable build is a transmission that matches the job. Whether you're hauling, cruising, or wheeling, the right transmission setup makes all the difference.
Between 1973 and 1987, Chevrolet and GMC offered a wide range of both automatic and manual transmissions in their C/K series trucks—each with unique strengths, weaknesses, and ideal use cases. From rugged SM465 4-speeds to smooth-shifting TH700R4 overdrives, this era of trucks saw serious evolution in driveline technology.
In this guide, we’ve compiled the most complete reference for factory Squarebody transmissions you’ll find online. You'll learn:
- Every automatic and manual transmission that came in Squarebody trucks
- What years each was offered and in what configurations
- How to identify your transmission using physical cues and casting numbers
- The pros and cons of each transmission type
- Common issues and wear points to watch for
- Which transmissions are best suited for towing, off-roading, daily driving, or swapping
This article was written to help owners, builders, and restorers get a clearer understanding of what they’re working with—and what options they have—before committing to a build path or driveline upgrade.
Let’s dig in.
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Factory Transmission Options for 1973–1987 Squarebody Trucks
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AUTOMATIC TRANSMISSIONS
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Name Type Speeds Years Used Notes
|----------------|----------|--------|-------------|------------------------------|
TH350 Auto 3 1973–1986 Lighter-duty, no overdrive
TH400 Auto 3 1973–1987 Heavy-duty, no overdrive
TH700R4 Auto 4 1982–1987 Overdrive, better MPG
TH200C Auto 3 1981–1984 Light-duty only
TH200-4R Auto 4 1984–1987 Rare in trucks
MANUAL TRANSMISSIONS
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Name Type Speeds Years Used Notes
|----------------|----------|--------|-------------|------------------------------|
SM465 Manual 4 1973–1987 “Granny gear” first gear
SM420 Manual 4 Up to 1977* Phased out early
NP435 Manual 4 1973–1979** Found in some GMC models
MY6 (A833) Manual 4 1981–1987 Overdrive 4th, rare option
Saginaw 3-Speed Manual 3 1973–1984 Column-shift base model
Borg Warner T-10 Manual 4 Rare Mostly performance models
*SM420s were factory-installed into early K-series trucks until about 1977.
**NP435 mostly found in heavier-duty and GMC variants.
📊 Transmission Breakdown: Pros, Cons & Common Issues
🔧 TH350 (Turbo Hydramatic 350)
- Type: Automatic, 3-speed
- Pros: Lightweight, reliable, easy to rebuild
- Cons: No overdrive, not ideal for towing
- Common Issues: Worn clutches, leaking seals, detent cable failure
🔧 TH400 (Turbo Hydramatic 400)
- Type: Automatic, 3-speed
- Pros: Strong, durable, excellent for towing or big block setups
- Cons: Heavy, uses more power, no OD
- Common Issues: Modulator failure, kickdown switch failure, fluid leaks
🔧 TH700R4
- Type: Automatic, 4-speed with overdrive
- Pros: Great MPG, lower highway RPM, wide gear ratio
- Cons: Early models (pre-1986) had reliability issues
- Common Issues: Weak 3-4 clutch pack, TV cable misadjustment, torque converter failure
🔧 SM465
- Type: Manual, 4-speed
- Pros: Incredibly strong, granny gear first, simple and rebuildable
- Cons: No overdrive, heavy, not smooth-shifting
- Common Issues: Worn synchros (esp. 2nd gear), shifter slop
🔧 Saginaw 3-Speed
- Type: Manual, 3-speed (column shift)
- Pros: Simple and cheap to maintain, found in base models
- Cons: Weak for heavy loads, no OD
- Common Issues: Sloppy linkage, gear grind, worn bushings
🔧 MY6 (NP A833 Overdrive)
- Type: Manual, 4-speed with OD 4th
- Pros: Manual transmission with OD, rare and fuel-efficient
- Cons: Weak for towing, rare parts
- Common Issues: Synchro wear, hard to find replacement parts
🔍 How to Identify Factory Transmissions
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Transmission Identification Tips
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1. VIN Decoding (Post-1981)
- VIN does NOT directly identify the transmission.
- Check glove box RPO codes, service records, or visual inspection.
2. Transmission Pan Shape (Automatics)
- TH350: Square pan with corner cut
- TH400: Larger, asymmetrical pan with “hump”
- 700R4: Rectangular pan with corner steps
3. Casting Numbers and Stamps
- Found on driver’s side bellhousing or tailshaft.
- Manuals often stamped with GM part numbers.
4. Shifter Location (Manuals)
- SM465: Top-mounted tower shifter
- Saginaw: Column shifter
- MY6: Side shifter, similar to car setups
5. Bellhousing Bolts
- Count and location can help ID early vs. later designs.
🔚 Closing Statement for the Article
If you're serious about preserving or upgrading your Squarebody, knowing your transmission is just as important as your engine. Whether you're running a bulletproof SM465 or a smooth-shifting 700R4, this guide was built to help you understand exactly what’s under your truck — and what it’s capable of.
We covered every transmission offered from the factory between 1973 and 1987, along with ID tips, weak spots, and upgrade paths to help you wrench with confidence.
For more squarebody tech, how-to’s, and real-world advice, stick with
Blazin’ Biddles Off-Road — your go-to spot for all things Squarebody.