Axle Swap and Upgrade Guide - Square Body Trucks

Overview

Axle swaps and upgrades are among the most common modifications performed on 1973-1987 GM Square Body trucks. Whether you're dealing with worn-out factory axles, need stronger components for increased towing capacity, or want to improve performance with different gear ratios, understanding your options is crucial for any Square Body owner or restorer.

This guide covers both front and rear axle upgrades, from simple differential swaps to complete axle replacements. Factory axles in these trucks ranged from the lightweight 10-bolt rear in base C10 models to the robust 14-bolt full-float in heavy-duty applications. Understanding your current setup and upgrade path will save time, money, and frustration.

Common reasons for axle upgrades include:
- Increased strength for towing or performance applications
- Better gear ratios for fuel economy or acceleration
- Limited slip or locking differentials for improved traction
- Wider track width for stability and tire clearance
- Disc brake conversions for improved stopping power

Common Issues

Factory Axle Weaknesses

10-Bolt Rear Axles (7.5" and 8.5")
- 7.5" 10-bolt: Weak ring gear, prone to failure with oversized tires or performance modifications
- 8.5" 10-bolt: Semi-float design limits load capacity, C-clip axle retention can fail catastrophically
- Both suffer from limited gear ratio availability and weak factory differentials

Front Axles
- 10-bolt front (8.5"): Adequate for most applications but limited aftermarket support
- Dana 44 front: Strong but narrow track width, expensive rebuild parts
- CV joints in 4WD models prone to wear with lifted suspensions

12-Bolt Rear Issues
- While stronger than 10-bolts, still semi-float design
- Limited to passenger car applications, not available in trucks from factory
- Often confused with 8.5" 10-bolt by inexperienced buyers

Parts & Materials Needed

Basic Tools Required

• Floor jack and jack stands (minimum 3-ton capacity)
• Socket set (SAE and metric, 3/8" and 1/2" drive)
• Torque wrench (80-200 ft-lbs range minimum)
• Gear pullers (3-jaw and slide hammer types)
• Axle pullers or slide hammer with axle adapters
• Differential spreader tool (for ring gear installation)
• Dial indicator (for backlash measurement)
• RTV silicone sealant and gear oil

Common Upgrade Axles

Rear Axle Options:

1. GM 14-Bolt Full-Float (10.5" ring gear)
   - Best sources: 1973-1987 C20/C30, 1988+ 3/4-ton trucks
   - Width: 65.5" (truck), 61.5" (van)
   - Bolt pattern: 8x6.5" (8x165mm)
   - Gear ratios: 3.73, 4.10, 4.56 common
   - Part numbers: Look for axle codes GT4, GT5, GU4, GU6

2. GM 12-Bolt (8.875" ring gear)
   - Sources: 1965-1972 A-body, 1967-1981 F-body, some trucks
   - Width: 58.5" (car), 62.5" (truck version)
   - Bolt pattern: 5x4.75" (5x120mm)
   - Stronger than 10-bolt but still semi-float

3. Ford 9-Inch
   - Width: Custom or 57" (Mustang), 59" (truck)
   - Bolt pattern: 5x4.5" or custom
   - Excellent aftermarket support
   - Companies: Currie, Moser, Strange offer complete assemblies

Front Axle Options:

1. Dana 60 (4WD)
   - Sources: 1977-1991 F-250/F-350, 1978-1993 Dodge trucks
   - Width: ~60" typical
   - Ultimate strength for heavy-duty applications

2. 10-Bolt Corporate (2WD)
   - Direct bolt-in for 2WD trucks
   - Disc brake conversion available
   - Part number prefix: 26008XXX series

Differential Options

Limited Slip Units:
- GM Posi-Traction: Part numbers vary by application
- Eaton TrueTrac: 912A549 (GM 8.5"), 912A586 (GM 14-bolt)
- Auburn Pro Series: Model-specific part numbers

Locking Differentials:
- Detroit Locker: DET-187SL-A (8.5"), DET-188SL-A (14-bolt)
- ARB Air Locker: RD117 (8.5"), RD132 (14-bolt)
- Yukon Zip Locker: Electric activation, model-specific

Step-by-Step Guide

Rear Axle Swap Procedure

Phase 1: Removal

1. Safety first: Park on level ground, engage parking brake, chock front wheels
2. Raise vehicle: Use proper jack points, support with jack stands under frame
3. Remove wheels and tires
4. Disconnect driveshaft: Mark relationship to pinion flange, remove U-bolts
5. Disconnect brake lines: Cap or plug to prevent fluid loss
6. Remove shock absorbers: Lower mounting bolts only
7. Support axle: Use floor jack under differential
8. Remove leaf spring U-bolts: Note orientation and hardware
9. Lower axle assembly: Carefully guide out from under vehicle

Phase 2: Installation

1. Test fit new axle: Check spring perch location and brake line routing
2. Transfer necessary components:
   - Brake backing plates or calipers
   - Emergency brake cables
   - Vent hoses
3. Position axle: Use floor jack, align with leaf springs
4. Install U-bolts:
   - Torque specification: 90 ft-lbs for 7/16" bolts, 120 ft-lbs for 1/2" bolts
   - Use thread locker on bolt threads
5. Connect driveshaft: Align marks, torque U-bolt nuts to 15 ft-lbs
6. Install brake lines: Use new copper washers, bleed system completely
7. Fill differential: Use manufacturer-specified gear oil and quantity
8. Install wheels: Torque lug nuts to specification (typically 100 ft-lbs)

Front Axle Swap (2WD)

Key Differences from Rear:
- Steering linkage: Must be disconnected at tie rod ends and center link
- Brake proportioning: May require adjusting valve or master cylinder
- Alignment: Professional alignment required after installation
- Spring mounting: Coil springs require different support procedures

Critical Measurements:
- Caster angle: 2-4 degrees positive typical
- Camber: 0.5 degrees positive maximum
- Toe-in: 1/8" to 3/16" total

Year/Model Differences

1973-1980 Models

Standard Equipment:
- C10: 7.5" 10-bolt rear, 10-bolt front
- C20: 8.5" 10-bolt or 14-bolt rear depending on GVW
- K10: 7.5" 10-bolt rear, Dana 44 front
- K20: 14-bolt rear, Dana 44 front

Identification:
- Square axle tubes on 10-bolt differentials
- Round tubes on 12-bolt and 14-bolt units
- Axle codes on glove box sticker: GU4 (3.73), GT4 (4.10), etc.

1981-1987 Models

Changes:
- Metric fasteners introduced on some components
- Different brake line fittings (some metric threads)
- Updated emergency brake cable routing
- Computer-controlled emissions may affect gear ratio choices

K5 Blazer Specifics:
- Shorter wheelbase affects driveshaft length requirements
- Full convertible top models have different body mount considerations
- Spare tire location under rear affects exhaust routing

Suburban Considerations

Unique factors:
- Longer wheelbase requires longer driveshafts
- Higher load capacity makes 14-bolt swap more beneficial
- Rear overhang affects departure angle with larger tires

Tips & Best Practices

Before You Start

Research Your Current Axle:
- Measure bolt pattern and width accurately
- Count ring gear bolts: 10 bolts = 10-bolt, 12 bolts = 12-bolt, etc.
- Check gear ratio: Divide driveshaft rotations by wheel rotations
- Verify spline count on axle shafts for parts compatibility

Budget Planning:
- Basic swap: $500-1500 for used axle and basic refresh
- Performance build: $2000-5000 with new gears, locker, axles
- Professional installation: Add $800-1500 labor costs

Installation Best Practices

Gear Setup Critical Points:
- Backlash: 0.005"-0.009" for most applications
- Pattern contact: 50-60% contact area on drive side
- Preload: Follow manufacturer specifications exactly
- Break-in: 500 miles easy driving, change gear oil

Common Mistakes to Avoid:
- Mixing gear oil types: Use specified GL-5 rating
- Overtightening ring gear bolts: Use proper sequence and torque
- Ignoring brake proportioning: Can cause dangerous brake imbalance
- Skipping alignment: Will cause tire wear and handling issues

Brake System Integration

Disc Brake Conversions:
- Master cylinder sizing: May need larger bore for disc brakes
- Proportioning valve: Required for front disc/rear drum combinations
- Brake line routing: Plan for proper mounting and protection

Emergency Brake:
- Cable length: Measure carefully, may need custom cables
- Adjustment: Ensure proper travel and holding power
- Inspection: Check for binding or interference with suspension travel

Troubleshooting

Noise Diagnosis

Whining Noise:
- Cause: Improper gear mesh or low fluid level
- Solution: Check backlash and fluid level, verify gear pattern
- Prevention: Professional gear setup, quality gear oil

Clicking in Turns:
- Cause: Worn CV joints (4WD) or limited slip clutches
- Solution: Replace CV joints or service differential
- Note: Normal for some locking differentials

Grinding or Growling:
- Cause: Worn bearings or damaged ring gear
- Solution: Complete differential overhaul required
- Warning: Stop driving immediately to prevent further damage

Vibration Issues

Driveline Vibration:
- Check U-joints: Replace if any play detected
- Verify driveshaft phasing: Ensure proper yoke alignment
- Measure driveshaft angles: Should be equal at transmission and axle

Wheel Vibration:
- Balance wheels and tires: Especially important with larger/heavier wheels
- Check axle runout: Should be less than 0.030" total indicator reading
- Inspect brake components: Warped rotors or drums cause vibration

Gear Ratio Verification

Calculation Method:
1. Jack up vehicle: Both rear wheels off ground
2. Mark tire and driveshaft: Use chalk or tape
3. Rotate driveshaft: Count complete revolutions
4. Turn wheel one revolution: Count driveshaft turns
5. Result: Driveshaft turns = gear ratio (approximately)

Common Ratios:
- 2.73: Highway/fuel economy
- 3.08: Balanced performance
- 3.42: Most common factory ratio
- 3.73: Good compromise
- 4.10: Performance/towing
- 4.56: Heavy duty/large tires

Professional vs. DIY Decision Points

DIY Appropriate:
- Complete axle swaps with existing gears
- Brake component installation
- Basic maintenance and fluid changes

Professional Recommended:
- Ring and pinion installation: Requires specialized tools and experience
- Bearing preload setup: Critical for longevity
- Custom driveshaft work: Length and balance critical
- Suspension geometry changes: Affects handling and tire wear