Overview
The 10-bolt rear axle was the standard rear differential configuration for most 1973-1987 GM Square Body trucks, including C10, C20, K10, K20, K5 Blazer, and Suburban models. Named for the ten bolts securing the rear cover, these axles came in several configurations with different ring gear sizes, gear ratios, and carrier types. Understanding your specific 10-bolt configuration is crucial for maintenance, upgrades, and parts replacement.
The primary 10-bolt variants found in Square Body trucks include:
- 7.5" (7.625") ring gear - Light duty, primarily C10/K10
- 8.5" (8.875") ring gear - Heavy duty, C20/K20 and some loaded C10s
- Corporate 14-bolt - Not a 10-bolt design, covered separately
This knowledge is essential for selecting correct replacement parts, planning differential upgrades, and understanding your truck's towing and performance capabilities.
Common Issues
Gear Oil Leaks
- Rear cover gasket failure - Most common leak point
- Pinion seal deterioration - Causes oil dripping from front of differential
- Axle seals - Oil leaking around axle tubes at wheels
Differential Noise
- Whining during acceleration - Often indicates worn ring and pinion
- Clunking on direction changes - Suggests worn spider gears or axles
- Grinding sounds - May indicate damaged bearings or severely worn gears
Limited Slip Clutch Wear
- Chatter during turns - Common with worn Positraction clutches
- One wheel spinning - Limited slip no longer functioning
Weak 7.5" Axle Limitations
- Axle shaft failure - Common with larger tires or heavy use
- Ring gear breakage - Occurs with excessive torque applications
Parts & Materials Needed
Identification Tools
- Flashlight - For reading axle codes
- Wire brush - Clean stamped codes on axle tube
- Measuring tape - Verify ring gear diameter
Common Replacement Parts
Gaskets and Seals
Rear Cover Gasket:
- 7.5": Fel-Pro RDS55018 or equivalent
- 8.5": Fel-Pro RDS55020 or equivalent
Pinion Seal:
- 7.5": National 710307 or Timken 710307
- 8.5": National 710308 or Timken 710308
Axle Seals:
- Standard: National 710142
- Heavy Duty: Timken 710142
Gear Oil
- 75W-90 Synthetic - Recommended for all applications
- 75W-140 - Heavy duty applications
- GM Limited Slip Additive - Part #1052358 for Positraction units
- Capacity: 3.5 quarts (7.5"), 4.2 quarts (8.5")
Upgrade Components
- Auburn Pro Series LSD - Modern limited slip upgrade
- Eaton Truetrac - Helical gear limited slip
- Mini spool - Racing applications only
Step-by-Step Guide
Axle Identification Process
Step 1: Locate Axle Code
- Jack up rear of truck and secure with jack stands
- Clean the axle tube on passenger side near differential
- Look for stamped code - Usually 2-3 letters followed by numbers
- Record complete code including any additional numbers
Step 2: Decode Axle Information
Common 7.5" Codes:
- BQ: 3.08 ratio, open differential
- BR: 3.42 ratio, open differential
- BS: 3.73 ratio, open differential
- BT: 4.10 ratio, open differential
Common 8.5" Codes:
- GT: 3.08 ratio, open differential
- GU: 3.42 ratio, open differential
- GV: 3.73 ratio, open differential
- GW: 4.10 ratio, open differential
Step 3: Physical Verification
- Remove rear cover - Count bolts (should be 10)
- Measure ring gear - 7.5" or 8.5" diameter
- Check for limited slip - Look for clutch packs beside spider gears
- Inspect gear condition - Note wear patterns and damage
Basic Maintenance Procedure
Step 1: Drain and Inspection
- Raise vehicle and support securely
- Clean around rear cover thoroughly
- Remove lower cover bolts first to allow draining
- Remove remaining bolts and cover
- Inspect gears and bearings for wear or damage
Step 2: Cleaning
- Clean cover and gasket surface with scraper
- Wipe differential housing with clean rags
- Check magnetic drain plug for metal particles
- Inspect cover for cracks or warping
Step 3: Reassembly
- Install new gasket - Use RTV sealant sparingly if needed
- Torque cover bolts to 20 ft-lbs in crossing pattern
- Fill with appropriate gear oil through fill plug
- Add limited slip additive if equipped (4 oz typical)
Year/Model Differences
1973-1980 Models
C10/K10 Light Duty
- Standard: 7.5" 10-bolt with 3.08 or 3.42 gears
- Heavy Duty Package: 8.5" 10-bolt available
- Positraction: Optional on both axle sizes
C20/K20 Heavy Duty
- Standard: 8.5" 10-bolt with 3.73 or 4.10 gears
- Corporate 14-bolt: Available on some heavy duty applications
- Full-floating axles: K20 models typically full-floating
1981-1987 Models
Updated Configurations
- Improved seals - Better quality axle and pinion seals
- Revised gear ratios - Some ratio changes for fuel economy
- Computer-controlled applications - Later models with ECM considerations
K5 Blazer Specific
- 7.5" standard through 1985
- 8.5" optional with towing package
- Full-time 4WD models may have different gear ratios
Suburban Applications
- 8.5" more common due to typical heavy-duty use
- Higher numerical ratios (3.73, 4.10) standard
- Limited slip frequently ordered for traction
Tips & Best Practices
Identification Best Practices
- Always verify physically - Don't rely solely on build sheets
- Check both axle tubes - Codes sometimes stamped on both sides
- Use GM parts books - Cross-reference codes with official documentation
- Take photos - Document codes before cleaning for future reference
Maintenance Excellence
- Change oil every 30,000 miles or 2 years minimum
- Use quality synthetic oil - Extends component life significantly
- Don't overfill - Check level at fill plug, not above
- Warm oil changes - Drain when warm for complete removal
Upgrade Considerations
- 7.5" axle limits - Maximum 33" tires with moderate use
- 8.5" preferred for any serious towing or off-road use
- Limited slip upgrade - Significantly improves traction
- Gear ratio matching - Consider transmission and tire size
Common Mistakes to Avoid
- Wrong oil type - Never use engine oil in differential
- Overtightening cover bolts - Can warp cover or strip threads
- Ignoring leaks - Small leaks become major failures
- Mixing limited slip additives - Use only GM-approved additives
Troubleshooting
Noise Diagnosis
Whining Noise
Symptoms: High-pitched whine during acceleration or deceleration
- Cause: Worn ring and pinion gears, incorrect backlash
- Solution: Professional gear set replacement and setup
- Prevention: Regular oil changes, avoid shock loading
Clunking Sounds
Symptoms: Metallic clunking when changing direction
- Cause: Worn spider gears, damaged axle splines, loose carrier
- Solution: Disassemble and inspect all internal components
- Immediate action: Avoid aggressive driving until repaired
Grinding Noise
Symptoms: Continuous grinding or rumbling
- Cause: Failed wheel bearings, damaged ring gear teeth
- Solution: Immediate professional inspection required
- Warning: Stop driving immediately - catastrophic failure possible
Leak Diagnosis
Pinion Seal Leak
Symptoms: Oil dripping from front of differential
- Test: Check u-joint area for oil accumulation
- Cause: Worn seal, damaged pinion flange, excessive backlash
- Solution: Replace seal, inspect flange surface, check preload
Axle Seal Leak
Symptoms: Oil on inside of wheels, wet brake shoes
- Test: Remove wheels, inspect brake components
- Cause: Worn seals, damaged axle surface, plugged vent
- Solution: Replace seals, repair axle surface, clear vent tube
Cover Gasket Leak
Symptoms: Oil seepage around rear cover perimeter
- Test: Clean area, drive truck, recheck for active leaks
- Cause: Bad gasket, warped cover, loose or overtightened bolts
- Solution: Replace gasket, check cover flatness, proper torque sequence
Performance Issues
Limited Slip Not Working
Symptoms: Inside wheel spins freely, no limited slip action
- Cause: Worn clutches, wrong oil, missing additive
- Test: Jack up rear, spin one wheel - other should turn opposite
- Solution: Rebuild limited slip or upgrade to modern unit
Gear Ratio Verification
Symptoms: Uncertain of actual gear ratio installed
- Test: Mark driveshaft and wheel, rotate wheel one complete turn
- Count: Number of driveshaft rotations = gear ratio
- Compare: Match results with axle code identification