Overview
The transmission swap guide for GM Square Body trucks (1973-1987) focuses on understanding the rare and desirable transmission options that came from the factory, particularly for collectors and enthusiasts looking to maintain authenticity or upgrade to period-correct performance transmissions. While most Square Body trucks came with common automatic transmissions like the TH350 and TH400, certain rare manual transmission options like the NP833 represent some of the most sought-after drivetrain components for these trucks.
This guide covers the identification, sourcing, and installation considerations for both common and rare transmission swaps, with particular emphasis on maintaining the authenticity of special edition and rare Square Body configurations.
Common Issues
- Rare transmission identification: Many owners are unaware they have a rare transmission option like the NP833
- Parts availability: Finding rebuild kits and replacement parts for rare transmissions can be extremely challenging
- Documentation gaps: Limited factory records make it difficult to verify original transmission specifications for special editions
- Modification history: Many Square Bodies have been modified over the decades, making it difficult to determine original drivetrain configurations
- Crossmember compatibility: Different transmissions may require specific crossmembers and mounting hardware
Parts & Materials Needed
Rare Factory Transmissions
- NP833 4-speed manual: Found in select Square Body trucks, particularly performance-oriented models
- Muncie 4-speed: Limited availability in early Square Body years
- SM465 4-speed: Heavy-duty manual option for 3/4-ton and 1-ton applications
Common Swap Candidates
- TH350: Light-duty automatic, common in 1/2-ton applications
- TH400: Heavy-duty automatic, standard in 3/4-ton and 1-ton trucks
- 700R4/4L60: Overdrive automatic (1982+ applications)
Supporting Components
- Transmission crossmember (varies by transmission type)
- Driveshaft modifications or replacement
- Clutch components (for manual swaps)
- Transmission cooler lines and fittings
- Shifter linkage and components
- Torque converter (automatic applications)
Step-by-Step Guide
Identification Process
- Locate transmission identification tag on the passenger side of the transmission case
- Document all stampings and codes before removal
- Cross-reference codes with GM service manuals to verify authenticity
- Photograph the installation for future reference
Pre-Swap Preparation
- Verify crossmember compatibility with target transmission
- Measure driveshaft length and note yoke specifications
- Check bellhousing bolt patterns for engine compatibility
- Assess cooling requirements for automatic transmissions
Installation Considerations
- Support the engine properly during transmission removal
- Mark driveshaft orientation before disconnection
- Drain all fluids and dispose of properly
- Clean all mating surfaces thoroughly
- Apply proper torque specifications to all fasteners
Year/Model Differences
Early Square Body (1973-1980)
- NP833 availability: Most common in performance applications during this period
- TH350/TH400 dominance: Standard automatic options
- SM465: Available in heavy-duty applications
Late Square Body (1981-1987)
- 700R4 introduction: Overdrive automatic became available in 1982
- NP833 discontinuation: Rare transmission phased out
- Computer controls: Later models may have electronic transmission controls
Special Edition Considerations
- Big 10 models: Factory big block 1/2-ton trucks may have unique transmission specifications
- Desert Fox, Sarge, Beau James: Special editions may retain rare transmission options
- Blazer Chalet: Unique drivetrain configurations possible
Tips & Best Practices
Authentication
- Verify numbers matching: Ensure transmission codes match factory records when possible
- Document provenance: Maintain records of transmission history and modifications
- Consult experts: Work with Square Body specialists for rare transmission identification
Installation Best Practices
- Use proper lifting equipment: Transmissions are heavy and awkward to handle
- Replace wear items: Install new seals, gaskets, and filters during swap
- Check fluid compatibility: Use correct transmission fluid specifications
- Test all functions: Verify proper operation before final assembly
Common Mistakes to Avoid
- Mixing transmission types: Don't assume all Square Body transmissions are interchangeable
- Ignoring cooling requirements: Automatic transmissions require adequate cooling
- Improper torque converter installation: Ensure proper engagement before installation
- Neglecting driveline angles: Maintain proper U-joint angles for longevity
Troubleshooting
Identification Issues
- Missing tags: Use case casting numbers and internal components for identification
- Conflicting information: Cross-reference multiple sources for verification
- Modified transmissions: Look for signs of internal modifications or rebuilds
Installation Problems
- Fitment issues: Verify correct crossmember and mounting hardware
- Driveshaft problems: Check length and yoke compatibility
- Shifting difficulties: Ensure proper linkage adjustment and fluid levels
- Cooling concerns: Monitor transmission temperatures during initial operation
Performance Issues
- Slipping: Check fluid level and condition first
- Hard shifting: Verify proper adjustment of TV cable (700R4) or vacuum modulator
- Noise: Investigate internal wear or improper installation
- Leaks: Check all seals and gasket surfaces for proper installation
Safety Warning: Always use proper jack stands and lifting equipment when working under vehicles. Transmission fluid and components can be extremely hot during operation. Dispose of all fluids according to local environmental regulations.
The rarity of certain transmission options in Square Body trucks, particularly the NP833 manual transmission, makes proper identification and preservation crucial for maintaining the authenticity and value of these increasingly collectible vehicles.