Rear Axle and Drivetrain Upgrades - Square Body Trucks

Overview

Rear axle and drivetrain upgrades are among the most impactful modifications for 1973-1987 GM Square Body trucks. Whether you're building a high-performance street truck, preparing for heavy towing, or simply replacing worn components, understanding your axle options and upgrade paths is crucial for maximizing performance, reliability, and safety.

Square Body trucks came with various rear axle configurations from the factory, ranging from lightweight 7.5" 10-bolts to heavy-duty 14-bolt full-floaters. This article covers identification, common upgrade paths, installation procedures, and the supporting drivetrain modifications needed to handle increased power and torque.

Why Upgrade?
- Increased strength for higher horsepower applications
- Better gear ratios for performance or fuel economy
- Improved traction with limited-slip or locking differentials
- Enhanced reliability under heavy loads or aggressive driving

Common Issues

Factory Axle Limitations

7.5" 10-Bolt (Corporate 10)
- Weak Points: Ring gear only 7.5", thin axle tubes, weak C-clip axle retention
- Power Limits: ~300 hp/300 lb-ft safely, prone to ring gear failure beyond this
- Common Failures: Stripped ring gear teeth, bent axle tubes, C-clip failures

8.5" 10-Bolt (Corporate 10)
- Weak Points: C-clip axle retention, limited gear ratio availability
- Power Limits: ~400 hp/400 lb-ft with quality build
- Common Failures: Axle shaft breakage at C-clip groove, carrier bearing wear

12-Bolt (Passenger Car Type)
- Weak Points: Smaller axle tubes than truck 12-bolt, limited aftermarket support
- Power Limits: ~500 hp with proper setup
- Common Failures: Axle tube flex, pinion bearing wear

Supporting Component Issues

Driveshaft Problems
- 1973-1980: Two-piece driveshaft with center support bearing failure
- 1981-1987: Single-piece shaft adequate for most applications but may need upgrading for high power
- Common Issues: U-joint failure, driveshaft vibration, inadequate strength for big block swaps

Transmission Compatibility
- TH350: Limited torque capacity (~350 lb-ft)
- TH400: Strong but poor overdrive options without expensive conversions
- 700R4: Good overdrive ratio but requires careful setup for reliability

Parts & Materials Needed

Popular Axle Upgrade Options

GM 12-Bolt Truck Axle

Corporate Axle Code: GT4, GT5, GU4, GU5, GU6
Ring Gear: 8.875" diameter
Ratios Available: 3.08, 3.42, 3.73, 4.10, 4.56
Axle Spline Count: 30-spline

GM 14-Bolt Full-Float

Corporate Axle Code: FF, FG, FK, etc.
Ring Gear: 9.5" diameter
Ratios Available: 3.42, 3.73, 4.10, 4.56, 5.13
Axle Spline Count: 30-spline (semi-float) or 35-spline (full-float)

Ford 9" (Popular Swap)

Housing Width: Custom required for proper fit
Ring Gear: 9" diameter
Ratios Available: Virtually unlimited aftermarket support
Axle Spline Count: 28, 31, 35-spline options

Essential Upgrade Components

Differential Options
- Open Differential: Stock configuration, adequate for street use
- Limited Slip: Eaton Posi Part: 19212411 (GM 12-bolt), Auburn Pro Part: 542065 (Ford 9")
- Locking Differential: Detroit Locker, ARB Air Locker, Eaton E-Locker

Axle Shafts
- Moser Engineering: Custom axles for most applications Part: A12GM35S-31 (12-bolt, 35-spline)
- Strange Engineering: High-strength options Part: S2214 (Ford 9", 31-spline)
- Yukon Gear: Budget-friendly upgrades Part: YA GM12-30 (12-bolt, 30-spline)

Driveshaft Upgrades
- 1310 Series: Standard strength, good to ~400 hp
- 1350 Series: Heavy-duty, good to ~600 hp
- 1480 Series: Maximum strength for extreme applications

Tools Required

Basic Installation
- Floor jack and jack stands (minimum 3-ton capacity)
- Socket set (SAE and metric)
- Torque wrench (80-200 lb-ft range)
- Gear puller set
- Slide hammer with axle puller attachment

Advanced Setup
- Dial indicator with magnetic base
- Inch-pound torque wrench
- Bearing race driver set
- Hydraulic press (20-ton minimum)
- Ring gear runout gauge

Step-by-Step Guide

Phase 1: Planning and Measurement

1. Identify Current Axle

Axle Code Location: Stamped on axle tube near differential
Cover Bolt Pattern: 10-bolt (square), 12-bolt (round), 14-bolt (round with 14 bolts)
Measure Housing Width: Flange-to-flange measurement

2. Determine Requirements
- Target horsepower/torque levels
- Intended use (street, strip, towing)
- Budget constraints
- Fabrication capabilities

3. Select Gear Ratio
Calculate optimal ratio based on:
- Tire diameter
- Transmission ratios
- Engine powerband
- Intended use

Phase 2: Axle Acquisition and Preparation

1. Source Replacement Axle

Junkyard Hunting Tips:
- 1973-1987 K20/K30: 14-bolt full-float axles
- 1973-1987 C20: 12-bolt truck axles (most desirable)
- 1967-1972 Pickup/Suburban: 12-bolt truck axles
- 1970-1981 Camaro/Nova: 12-bolt passenger car axles

2. Housing Preparation

# Typical housing width modifications needed:
Stock Square Body Width: ~62.5"
GM 12-bolt truck: ~60.5" (needs narrowing or spacers)
GM 14-bolt: ~65.5" (needs narrowing)
Ford 9": Custom width required

3. Differential Setup

Ring and Pinion Installation:
- Set pinion depth using crush sleeve or solid spacer
- Establish proper backlash (.006"-.010" typical)
- Set bearing preload per manufacturer specifications
- Verify contact pattern with marking compound

Phase 3: Installation

1. Remove Stock Axle
- Support vehicle securely on jack stands
- Disconnect brake lines and parking brake cables
- Remove driveshaft (mark orientation)
- Support axle with floor jack
- Remove leaf spring U-bolts
- Lower axle assembly

2. Install Upgraded Axle
- Position new axle under vehicle
- Install leaf spring plates and U-bolts
- Torque Specifications:
Leaf Spring U-bolts: 85-120 lb-ft (depending on thread size) Shock Absorber Bolts: 65 lb-ft Brake Line Fittings: 15-20 lb-ft

3. Driveshaft Modifications
- Measure new driveshaft length required
- Verify U-joint compatibility
- Balance driveshaft if length changed significantly

Phase 4: Supporting Modifications

1. Brake System Upgrades
- Larger axles often require different brake components
- Upgrade to larger drums or disc brake conversion
- Ensure proper brake line routing and length

2. Suspension Considerations
- Heavier axles may require spring rate adjustments
- Consider shock absorber upgrades for improved control
- Anti-wrap bars for high-torque applications

Year/Model Differences

Early Years (1973-1980)

Standard Equipment:
- C10: 7.5" 10-bolt standard, 8.5" 10-bolt optional
- C20: 12-bolt truck axle standard
- K10: 8.5" 10-bolt standard
- K20: 14-bolt full-float standard

Unique Considerations:
- Two-piece driveshaft standard (problematic center support bearing)
- Older brake systems with different mounting provisions
- Different spring perch designs

Later Years (1981-1987)

Standard Equipment:
- C10: 7.5" 10-bolt standard, 8.5" 10-bolt with tow package
- C20: 14-bolt semi-float became more common
- K10: 8.5" 10-bolt standard
- K20: 14-bolt full-float standard

Improvements:
- Single-piece driveshaft standard
- Improved brake systems
- More consistent spring perch designs

Model-Specific Notes

K5 Blazer/Suburban:
- Shorter wheelbase affects driveshaft angles
- Full-size spare tire considerations with larger axles
- Different exhaust routing may interfere with some swaps

C20/K20 Heavy Duty:
- Often came with desirable axle ratios from factory
- Stronger leaf springs better suited for axle upgrades
- May have different brake proportioning requirements

Tips & Best Practices

Selection Guidelines

Power Level Recommendations:

Up to 300 hp: Stock 8.5" 10-bolt acceptable
300-450 hp: GM 12-bolt truck axle recommended
450+ hp: 14-bolt or Ford 9" required
Extreme builds: Custom 9" or Dana 60

Gear Ratio Selection:
- 3.08: Highway cruising, overdrive transmissions
- 3.42-3.73: Good all-around ratios
- 4.10: Performance applications, non-overdrive transmissions
- 4.56+: Drag racing, extreme performance

Cost-Effective Approaches

Budget Build:
1. Source used 12-bolt truck axle
2. Refresh with new bearings and seals
3. Install limited-slip differential
4. Upgrade to stronger axle shafts

Premium Build:
1. New aftermarket housing (Strange, Moser)
2. Custom gear ratio and differential
3. Premium axle shafts and hardware
4. Professional setup and installation

Common Mistakes to Avoid

Installation Errors:
- Incorrect pinion depth setting (causes premature failure)
- Inadequate bearing preload (causes noise and wear)
- Poor brake line routing (interference or inadequate length)
- Ignoring driveshaft angles (causes vibration)

Component Mismatching:
- Using weak axle shafts with strong differential
- Inadequate U-joint strength for power level
- Mismatched gear ratios between front and rear (4WD models)

Troubleshooting

Noise Diagnosis

Whining Noise:
- Cause: Incorrect backlash, worn bearings, or inadequate lubrication
- Solution: Check gear setup, replace bearings, verify fluid level and type

Clunking on Acceleration:
- Cause: Worn U-joints, loose yoke, or differential wear
- Solution: Inspect and replace U-joints, check pinion nut torque

Grinding During Turns:
- Cause: Limited-slip differential wear or incorrect fluid
- Solution: Service differential, use proper friction modifier additive

Performance Issues

Poor Acceleration:
- Cause: Gear ratio too tall for application
- Solution: Install numerically higher gear ratio

Poor Fuel Economy:
- Cause: Gear ratio too low, or driveline inefficiency
- Solution: Consider overdrive transmission or taller gears

Vibration Problems:
- Cause: Driveshaft imbalance, incorrect angles, or worn components
- Solution: Balance driveshaft, adjust pinion angle, replace worn parts

Maintenance Requirements

Break-in Procedure:
1. Drive first 500 miles conservatively
2. Change gear oil after break-in period
3. Inspect for leaks and proper operation
4. Retorque all fasteners after initial driving

Ongoing Maintenance:
- Check gear oil level every 5,000 miles
- Change gear oil every 30,000 miles (severe service: 15,000 miles)
- Inspect U-joints every 10,000 miles
- Monitor for leaks and unusual noises

Recommended Fluids:

GM 10-bolt/12-bolt: 75W-90 synthetic gear oil
GM 14-bolt: 75W-90 synthetic gear oil
Limited-slip additive: GM Part #1052358 or equivalent
Capacity: 2.5-4.5 quarts depending on axle type

This comprehensive approach to rear axle and drivetrain upgrades will provide Square Body owners with the knowledge needed to make informed decisions and execute successful modifications that enhance performance, reliability, and driving enjoyment.