K-Series Truck Axle Configurations - Square Body Trucks

Overview

The K-Series (4WD) Square Body trucks from 1973-1987 utilized a variety of axle configurations depending on the model year, truck weight rating, and intended use. Understanding these configurations is crucial for Square Body owners planning modifications, repairs, or restorations. The axle setup directly affects performance, load capacity, parts availability, and modification potential.

GM used different axle combinations throughout the production run, with significant changes occurring in 1977 and 1980. The most common configurations include the 10-bolt corporate front axle paired with various rear axles including the 10-bolt 8.5", 12-bolt 8.875", and 14-bolt full-float depending on the application.

This knowledge is essential when:
- Replacing axle components or upgrading differentials
- Planning gear ratio changes
- Troubleshooting driveline vibrations or noise
- Determining load capacity and towing limits
- Sourcing replacement parts

Common Issues

Axle Identification Problems

Mismatched gear ratios between front and rear axles causing transfer case binding
Incorrect axle identification leading to wrong parts ordering
Mixed axle components from previous owner modifications

Mechanical Issues

10-bolt front axle weakness in heavy-duty applications
CV joint failures in 1980+ independent front suspension models
Ring and pinion wear from mismatched ratios or lack of maintenance
Carrier bearing failures in 12-bolt and 14-bolt rear axles
Axle seal leaks causing differential fluid loss

Transfer Case Related

Chain stretch and failure in NP208 transfer cases (1981-1987)
Pump rub issues in NP205 transfer cases with incorrect driveline angles

Parts & Materials Needed

Identification Tools

Axle code decoder chart (available from LMC Truck catalog)
Gear ratio identification kit or tooth counting method
Magnetic drain plug for differential service

Common Replacement Parts

Front Axle Components (10-Bolt Corporate)

Ring and pinion sets: GM 12471395 (3.73:1), GM 12471396 (4.10:1)
Carrier bearings: Timken LM501349/LM501310
Axle seals: National 710623 (inner), National 710624 (outer)
U-joints: Spicer 297X (1973-1979), Spicer 5-297X (1980-1987 CV)

Rear Axle Components

8.5" 10-Bolt:
- Ring and pinion: Richmond Gear 79-0023-1 (3.73:1)
- Posi unit: Eaton 19559-010

8.875" 12-Bolt:
- Ring and pinion: Richmond Gear 69-0058-1 (3.73:1)
- Posi unit: Eaton 19695-010

9.5" 14-Bolt Full-Float:
- Ring and pinion: Richmond Gear 79-0070-1 (4.10:1)
- Detroit Locker: Eaton 187SL-4

Tools Required

Bearing puller set
Dial indicator for backlash measurement
Torque wrench (0-200 ft-lbs capacity)
Gear marking compound
Axle pullers (slide hammer type)

Step-by-Step Guide

Axle Identification Process

Step 1: Locate Axle Codes

Front axle: Check the differential cover or axle tube for stamped codes
Rear axle: Look for RPO codes on the glove box label or Service Parts ID label
Transfer case: Identify by case shape and shift pattern

Step 2: Decode Axle Information

Front Axle Codes (Corporate 10-Bolt):

Code Format: XYZABC
X = Month (1-12)
Y = Day (01-31)  
Z = Year (3-7 for 1973-1987)
ABC = Gear ratio and options

Rear Axle RPO Codes:
- GT4: 8.5" 10-bolt, 3.08:1 ratio
- GT5: 8.5" 10-bolt, 3.42:1 ratio
- GU4: 8.875" 12-bolt, 3.07:1 ratio
- GU6: 8.875" 12-bolt, 3.73:1 ratio
- HC4: 9.5" 14-bolt full-float, 4.10:1 ratio

Step 3: Verify Gear Ratios

Jack up the vehicle and secure with jack stands
Mark the driveshaft and tire with chalk
Rotate the tire exactly two complete turns
Count driveshaft rotations (3.73 turns = 3.73:1 ratio)

Gear Ratio Matching Procedure

Step 1: Determine Target Ratio

Highway driving: 3.08:1 to 3.42:1
Mixed use: 3.73:1 to 4.10:1
Heavy towing/off-road: 4.56:1 to 4.88:1

Step 2: Calculate Costs

Ring and pinion sets: $300-500 per axle
Installation labor: $400-600 per axle
Supporting components: $200-400 per axle

Year/Model Differences

1973-1976 Models

Front axle: Corporate 10-bolt with leaf springs
Rear axle: 10-bolt 8.5" (K10) or 12-bolt 8.875" (K20)
Transfer case: NP205 full-time 4WD (chain-driven)
Common ratios: 3.07:1, 3.73:1, 4.10:1

1977-1979 Models

Front axle: Corporate 10-bolt with leaf springs
Rear axle: Same as 1973-1976
Transfer case: NP205 part-time 4WD introduced
New option: Factory locking differential available

1980 Model Year

Major changes for K10/K5 Blazer:
- Independent Front Suspension (IFS) introduced
- Front axle: Corporate 10-bolt with CV joints
- Rear axle: Continues 10-bolt 8.5" (K10) or 12-bolt 8.875" (K20)
- Transfer case: NP208 aluminum case introduced

1981-1987 Models

K10/K5 Blazer: IFS with 10-bolt front, 10-bolt 8.5" rear
K20/K2500: Solid axle front, 14-bolt 9.5" full-float rear standard
K30/K3500: 14-bolt front and rear (1982+)
Transfer case: NP208 standard, NP205 available through 1986

Suburban Specific Configurations

K10 Suburban (1973-1979): Same as K10 pickup
K20 Suburban (1973-1987): 12-bolt rear standard through 1980, 14-bolt 1981+
No IFS option: All Suburbans retained solid front axles through 1987

Tips & Best Practices

Axle Identification

Always verify ratios mechanically - RPO codes can be incorrect due to previous modifications
Check both axles - mismatched ratios are common in modified trucks
Document everything - Take photos of codes and measurements for future reference

Gear Ratio Selection

Consider tire size - Larger tires effectively lower gear ratios
Match driving habits - Highway drivers benefit from numerically lower ratios
Plan for modifications - Choose ratios that work with planned engine/transmission swaps

Installation Best Practices

Always replace both axles when changing ratios to ensure perfect matching
Use quality parts - Richmond Gear, Yukon, and Motive Gear offer reliable options
Professional installation recommended - Proper setup requires specialized tools and experience
Break-in period critical - Follow manufacturer's break-in procedures exactly

Cost-Effective Approaches

Complete axle swaps often cheaper than rebuilding existing axles
Salvage yard sourcing - Many Square Body trucks in yards with good axles
Group purchases - Coordinate with other owners for better pricing on gear sets

Troubleshooting

Gear Ratio Mismatch Symptoms

Symptoms:
- Transfer case binding or jumping out of 4WD
- Excessive tire scrubbing in turns
- Premature transfer case failure

Diagnosis:
1. Measure both axle ratios using the tire rotation method
2. Calculate difference - More than 0.1:1 difference causes problems
3. Check transfer case fluid for metal particles indicating damage

Solutions:
- Change one axle to match the other
- Change both axles to desired ratio
- Replace transfer case if damage occurred

Axle Noise Diagnosis

Whining noise:
- Constant whine: Usually ring and pinion wear
- Whine in turns: Likely differential spider gear wear
- Whine under load: Ring and pinion backlash or wear

Clicking/grinding:
- CV joints (IFS models): Click in turns, especially when cold
- U-joints: Click during acceleration/deceleration
- Wheel bearings: Grinding that changes with steering input

Vibration Issues

Driveline vibrations:
1. Check U-joints for wear or binding
2. Verify driveshaft phasing - Yokes must be in proper alignment
3. Measure driveline angles - Should be 1-3 degrees at each end
4. Check for bent driveshafts - Should run true within 0.010"

Recommended shops:
- Driveline Service of Portland (driveshaft balancing)
- Local machine shops often handle driveshaft work
- 4WD specialty shops for complex diagnosis

Parts Availability Issues

NLA (No Longer Available) parts:
- Some 12-bolt carriers discontinued - check Ratech and Strange Engineering
- 14-bolt front axle parts scarce - Corporate 14-bolt good upgrade option
- Transfer case parts - Advanced Adapters stocks hard-to-find components

Alternative sources:
- LMC Truck: Best selection of restoration-quality parts
- Classic Industries: Good pricing on common components
- Summit Racing: Performance and heavy-duty options
- Local dealerships: Still stock some common wear items