Bed Floor and Side Replacement - GM Square Body Trucks (1973-1987)
Overview
Bed floor and side replacement is one of the most common restoration projects for GM Square Body trucks (C10, C20, K10, K20, K5 Blazer, and Suburban). After 35+ years of service, the truck bed components are frequently compromised by rust, corrosion, and physical damage from hauling cargo. This comprehensive guide covers the complete replacement of bed floors, bed sides, tailgates, and associated hardware.
The bed replacement process involves removing the existing bed components, addressing any frame rust, and installing new reproduction or OEM parts. This project typically requires intermediate to advanced mechanical skills and can take 15-25 hours depending on the condition of the existing bed and chosen approach.
Why This Matters:
- Restores structural integrity for safe hauling
- Prevents further rust spread to cab and frame
- Significantly improves vehicle appearance and value
- Allows inspection and repair of hidden frame areas
Common Issues
Rust and Corrosion Patterns
Floor Pan Deterioration:
- Corner rot at rear wheel wells (90% of trucks affected)
- Front bed mount areas where moisture collects
- Tailgate contact zones from repeated impact
- Drain plug areas in 8-foot beds
Bed Side Problems:
- Lower rear corners behind wheel wells
- Stake pocket deterioration from galvanic corrosion
- Inner wheelhouse separation from outer skin
- Fuel filler area rust (passenger side)
Frame-Related Issues:
- Rear crossmember rot requiring replacement before bed installation
- Bed mounting bracket corrosion
- Frame rail deterioration at bed mount points
Physical Damage
- Dented or creased bed sides from cargo damage
- Bent tailgate frames from dropping heavy loads
- Cracked welds at stress points
- Missing or damaged hardware (tailgate chains, latches, etc.)
Parts & Materials Needed
Primary Components
Bed Floor Assemblies:
Short Bed (6.5'):
- LMC Truck: 34-4073 (1973-1987)
- Classic Industries: BF6587
Long Bed (8'):
- LMC Truck: 34-4074 (1973-1987)
- Classic Industries: BF8587
Bed Sides (Left/Right pairs):
Short Bed Sides:
- LMC Truck: 34-4075/34-4076
- Brothers Trucks: GM-BS-6587-L/R
Long Bed Sides:
- LMC Truck: 34-4077/34-4078
- Brothers Trucks: GM-BS-8587-L/R
Tailgates:
Standard Tailgate:
- LMC Truck: 34-4080 (1973-1987)
- Part Number: 14045492 (GM OEM)
Step-Side Tailgate:
- LMC Truck: 34-4081 (1973-1987)
Hardware and Fasteners
Bed Mounting Hardware:
- Bed bolts: 3/8" x 1.25" Grade 8 bolts (8 required)
- Flat washers: 3/8" zinc-plated (16 required)
- Lock washers: 3/8" split-ring (8 required)
- Bed strips: Rubber mounting strips between bed and frame
Tailgate Hardware:
Tailgate Chain Kit: LMC 34-4085
Tailgate Latch Assembly: LMC 34-4087
Tailgate Hinge Kit: LMC 34-4088
Tools Required
Essential Tools:
- Impact gun with 1/2" and 9/16" sockets
- Cut-off wheel or reciprocating saw
- MIG welder (minimum 140-amp capacity)
- Angle grinder with cutting and grinding discs
- Engine hoist or bed dolly for handling
- Torque wrench (50-100 ft-lb range)
Specialty Tools:
- Spot weld cutter for clean panel separation
- Panel clamps for alignment during welding
- Bed lifting brackets (LMC 99-4012)
Materials and Supplies
Welding Supplies:
- ER70S-6 welding wire (0.030" diameter)
- C25 shielding gas (75% Argon, 25% CO2)
- Weld-through primer for joint protection
Rust Prevention:
- POR-15 or Corroseal rust preventive paint
- Ospho phosphoric acid rust treatment
- 3M 08008 Body Schutz undercoating
- Bed liner material (spray-in or drop-in)
Step-by-Step Guide
Phase 1: Removal and Assessment
Step 1: Bed Removal
1. Disconnect tailgate chains and remove tailgate
2. Remove bed mounting bolts (8 total - 4 per side at frame rails)
3. Disconnect wiring for tail lights and fuel sender (if equipped)
4. Lift bed assembly using engine hoist or 4-person crew
- Weight: Short bed ~300 lbs, Long bed ~400 lbs
- Lifting points: Use bed rail reinforcement areas
Step 2: Component Separation
1. Cut spot welds connecting bed sides to floor
- Location: Every 2-3 inches along bottom flange
- Tool: 3/8" spot weld cutter bit
2. Remove tailgate mounting hardware
3. Separate bed sides from floor assembly
4. Document damage and measure replacement needs
Step 3: Frame Inspection and Repair
1. Inspect rear crossmember for structural integrity
2. Check bed mounting points for rust or damage
3. Replace crossmember if more than 25% compromised
4. Treat surface rust with phosphoric acid solution
5. Prime exposed metal with weld-through primer
Phase 2: Floor Installation
Step 4: Floor Preparation
1. Test fit new floor against frame rails
2. Mark drain hole locations (8-foot beds only)
3. Apply weld-through primer to all mating surfaces
4. Install rubber bed strips on frame rails
Step 5: Floor Mounting
1. Position floor assembly using alignment jig or measurements
- Front-to-back position: 1/2" gap from cab
- Side-to-side: Centered on frame rails
2. Install bed bolts finger-tight initially
3. Check diagonal measurements for square alignment
4. Torque bed bolts to 35 ft-lbs in cross pattern
Phase 3: Side Panel Installation
Step 6: Side Panel Preparation
1. Check panel fitment against installed floor
2. Adjust flanges if necessary for proper alignment
3. Apply weld-through primer to all joint areas
4. Install tailgate hardware before final welding
Step 7: Welding Sequence
1. Clamp panels in position using panel clamps
2. Tack weld every 6 inches to maintain alignment
3. Complete seam welding in 2-3 inch segments
- Settings: 18-20 volts, 180-200 IPM wire speed
- Technique: Push angle, 10-15 degree travel angle
4. Allow cooling between segments to prevent warpage
Step 8: Finish Welding
1. Grind weld seams flush with surrounding metal
2. Apply seam sealer to all joints
3. Prime welded areas immediately after grinding
4. Install stake pockets and bed rail caps
Phase 4: Final Assembly
Step 9: Hardware Installation
1. Install tailgate hinges with 15 ft-lb torque spec
2. Adjust tailgate alignment for even gaps
3. Install tailgate chains with proper length adjustment
4. Test tailgate operation and latch engagement
Step 10: Protective Coatings
1. Clean all surfaces with degreaser
2. Apply rust preventive primer to underside
3. Install bed liner or apply protective coating
4. Reinstall tail light assemblies
Year/Model Differences
1973-1980 vs 1981-1987 Variations
Fuel System Differences:
- 1973-1980: Side-mounted fuel tank with bed-mounted filler
- 1981-1987: Frame-mounted tank with cab-mounted filler
- Impact: Earlier trucks require fuel filler integration in passenger bed side
Tail Light Configurations:
1973-1980: Round tail lights
- Mounting: 4.5" diameter holes
- Wiring: 3-wire system
1981-1987: Rectangular tail lights
- Mounting: Rectangular cutouts
- Wiring: 4-wire system with separate turn signals
Model-Specific Considerations
C10/K10 vs C20/K20:
- Bed dimensions: Identical between half-ton and three-quarter ton
- Mounting hardware: Same bolt pattern and specifications
- Load capacity: Bed construction identical despite different frame ratings
Step-Side vs Fleet-Side:
- Step-side: Separate running boards, different side panel configuration
- Fleet-side: Integrated design, more common reproduction availability
- Interchange: Not compatible - completely different mounting systems
K5 Blazer Considerations:
- Shortened bed: Uses modified short-bed components
- Tailgate: Specific to Blazer, not interchangeable with pickup
- Spare tire mount: Integrated into bed floor design
Suburban Applications:
- Cargo floor: Similar construction but different dimensions
- Side panels: Taller configuration for passenger area
- Parts availability: More limited reproduction options
Tips & Best Practices
Professional Techniques
Welding Best Practices:
- Stitch weld long seams to prevent heat distortion
- Use backing plates on critical structural joints
- Maintain consistent travel speed for uniform penetration
- Clean base metal to bright steel before welding
Alignment Strategies:
- Build jigs from 2x4 lumber for consistent positioning
- Use laser level for bed rail straightness verification
- Check measurements at multiple points during assembly
- Test fit tailgate before final side panel welding
Cost-Saving Approaches
Budget-Friendly Options:
- Patch panels for localized rust repair vs full replacement
- Used OEM parts from salvage yards in dry climates
- DIY bed liner application vs professional spray-in
- Gradual replacement - floor first, sides later
Premium Approaches:
- EDP-coated reproduction parts for maximum corrosion resistance
- Professional media blasting of frame components
- Spray-in bed liner with lifetime warranty
- Stainless steel hardware for all fasteners
Common Mistakes to Avoid
Critical Errors:
- Skipping frame inspection - hidden rust will continue spreading
- Improper bed alignment - causes tailgate fit issues
- Insufficient rust treatment - premature failure of new components
- Over-welding - causes heat distortion and panel warpage
Quality Issues:
- Using wrong primer - standard primer vs weld-through primer
- Inadequate cleaning - oil and dirt contamination in welds
- Rushed timeline - proper curing time for primers and sealers
- Missing drain holes - water accumulation leads to premature rust
Troubleshooting
Fit and Alignment Issues
Problem: Tailgate won't close properly
- Cause: Bed sides not square or tailgate hinges misaligned
- Solution: Check diagonal measurements, adjust hinge mounting
- Specification: Gap should be 3-5mm around tailgate perimeter
Problem: Bed sides don't align with floor
- Cause: Floor not properly positioned or frame damage
- Solution: Verify bed bolt torque, check frame straightness
- Tool: Use bed alignment jig or string line method
Problem: Water pooling in bed corners
- Cause: Inadequate welding or missing drain provisions
- Solution: Drill 1/4" drain holes at low points, seal all joints
- Location: Rear corners and front bed mount areas
Welding Problems
Problem: Burn-through on thin metal
- Cause: Excessive heat or incorrect technique
- Solution: Reduce voltage, increase travel speed, use pulse settings
- Settings: 16-18 volts for 20-gauge reproduction panels
Problem: Poor weld penetration
- Cause: Contaminated base metal or insufficient heat
- Solution: Clean to bright metal, increase voltage 2-3 volts
- Verification: Cross-section test welds on scrap material
Problem: Weld porosity
- Cause: Moisture, rust, or shielding gas issues
- Solution: Preheat panels, verify gas flow rate (25-30 CFH)
- Prevention: Store welding wire in dry environment
Rust Recurrence
Problem: New rust appearing within 1-2 years
- Cause: Inadequate surface preparation or moisture intrusion
- Solution: Strip to bare metal, apply proper rust preventive system
- System: Ospho treatment → POR-15 → topcoat
Problem: Galvanic corrosion at stake pockets
- Cause: Dissimilar metals without proper isolation
- Solution: Apply dielectric compound, use stainless fasteners
- Prevention: Regular inspection and maintenance schedule
Problem: Frame rust continuing to spread
- Cause: Incomplete frame treatment during bed replacement
- Solution: Remove bed, treat frame comprehensively
- Method: Media blast → phosphoric acid → primer → paint system
This article represents established knowledge and best practices for GM Square Body truck bed restoration. Always consult service manuals and follow proper safety procedures when performing these repairs.