Setting backlash on a differential is a critical process that ensures the proper engagement and longevity of the ring and pinion gears, preventing premature wear and excessive noise.
Understanding Differential Backlash
Backlash refers to the small amount of clearance, or "play," between the meshing teeth of the ring and pinion gears. This clearance is essential; without it, the gears would bind, generate excessive heat, and fail prematurely. Too much backlash, however, can lead to gear whine, poor power transfer, and eventual gear damage. Achieving the correct backlash is a precise adjustment involving the differential carrier's position relative to the pinion.
Why Correct Backlash is Crucial
- Longevity: Proper clearance prevents the gear teeth from wearing out quickly.
- Noise Reduction: Incorrect backlash is a common cause of gear whine, especially during acceleration or deceleration.
- Efficiency: Optimizes the transfer of power from the driveshaft to the axles.
- Heat Management: Prevents excessive friction and heat buildup within the differential.
Tools You'll Need
Before beginning, gather the necessary tools:
- Dial Indicator with Magnetic Base: Essential for accurately measuring backlash.
- Torque Wrench: For tightening carrier bearing caps and other fasteners to specification.
- Shim Kit: Contains various thicknesses of shims for adjusting the carrier position.
- Bearing Race Driver Kit: For installing new bearing races (if replacing bearings).
- Bearing Press: For pressing new bearings onto the differential carrier (if replacing bearings).
- Seal Pullers and Installers: For axle seals and pinion seals.
- Impact Wrench (optional): For initial disassembly.
- Shop Manual: Specific to your vehicle's axle type, providing torque specifications and backlash ranges.
- Assembly Lube or Gear Oil: To lubricate components during assembly.
How to Set Backlash: A Step-by-Step Guide
The process involves measuring the existing backlash and then adjusting the differential carrier's position using shims until the optimal clearance is achieved.
1. Disassembly and Inspection
- Remove Differential Cover: Drain the old gear oil and remove the differential cover.
- Remove Axle Shafts (if full-floating): On some designs, axle shafts need to be removed to allow for carrier removal.
- Remove Carrier Bearing Caps: Loosen and remove the bolts securing the carrier bearing caps. Keep track of which cap goes on which side, as they are often machined as a set and must be reinstalled in their original positions.
- Remove Differential Carrier: Carefully lift the differential carrier out of the housing. This may require gently prying or wiggling it free.
- Inspect Gears and Bearings: Thoroughly inspect the ring and pinion gears for wear, pitting, or damage. Check the carrier bearings and races for any signs of wear. Replace any damaged components. If replacing carrier bearings, you'll need to press off the old ones and press on new ones, which will necessitate re-shimming.
2. Initial Carrier Installation and Shim Placement
When re-installing the carrier (especially after replacing shims or bearings), it's crucial to get a snug fit.
- Install Starting Shims: Begin with a combination of shims that allows the carrier to be installed into the housing with some resistance, but without excessive force. The goal is to establish a starting point for adjustment.
- Install Carrier Bearings and Races: Ensure all bearings and races are properly seated.
- Install Carrier Bearing Caps: Place the caps back in their original positions and finger-tighten the bolts. Do not fully torque them yet.
3. Measuring Backlash
This is where the precision comes in.
- Mount Dial Indicator: Attach the dial indicator's magnetic base securely to the differential housing. Position the indicator's stem so its tip rests perpendicular to a tooth on the ring gear.
- Zero the Indicator: Gently push the ring gear in one direction to take up all play, then set the dial indicator to zero.
- Measure Play: Gently rock the ring gear back and forth, allowing the dial indicator to show the amount of free movement. You'll want to do this in three separate locations on the ring gear to check for runout and consistency. This variation across different points helps ensure the gear is perfectly concentric.
- Record Readings: Note the backlash readings from all three locations. Your goal is for these readings to be consistent and within the manufacturer's specified range.
4. Adjusting Backlash with Shims
Adjusting backlash involves adding or removing shims from behind the carrier bearings to move the entire differential carrier assembly closer to or further away from the pinion gear.
- If the backlash is too tight (too little play): Move the carrier away from the pinion. This is achieved by removing shims from the side of the carrier closest to the pinion and adding an equivalent thickness of shims to the opposite side (further from the pinion).
- If the backlash is too wide (too much play): Adjust the carrier toward the pinion. This is done by removing shims from the side of the carrier further from the pinion and adding an equivalent thickness of shims to the side closest to the pinion.
Important Note: When adjusting, maintain the total shim pack thickness (or preload) to ensure proper bearing preload. If you remove 0.005 inches from one side, add 0.005 inches to the other side. This shifts the carrier without altering the bearing preload. Some differentials use adjustable collars instead of shims for this adjustment.
5. Checking Carrier Bearing Preload
After adjusting backlash, it's crucial to check the carrier bearing preload. This ensures the bearings are properly seated and have the correct amount of tension.
- Torque Carrier Bearing Caps: Torque the carrier bearing cap bolts to the manufacturer's specifications.
- Measure Rotational Torque: Use a beam-type torque wrench on one of the carrier bearing cap bolts or a special tool to measure the rotational torque required to turn the carrier. This measurement indicates the bearing preload. Consult your service manual for the correct preload specification (usually measured in inch-pounds). Adjust shims (adding or removing total thickness) if preload is incorrect, then re-check backlash.
6. Verify Gear Mesh Pattern
Setting backlash is only half the battle; the other half is ensuring a proper gear mesh pattern. This tells you how the ring and pinion teeth are contacting each other.
- Apply Gear Marking Compound: Apply a thin, even coat of gear marking compound to several teeth on the ring gear.
- Rotate Gears: Rotate the ring gear several full revolutions in both directions while applying resistance to the pinion (e.g., holding the driveshaft flange).
- Analyze Pattern: Observe the pattern left by the compound on the ring gear teeth.
- Ideal Pattern: The contact pattern should be centered on the tooth, extending from the root (inner part of the tooth) to the face (outer part) and slightly biased toward the toe (smaller end of the tooth).
- Incorrect Patterns:
- High (Toe-Heavy): Pinion needs to be moved deeper into the housing.
- Low (Heel-Heavy): Pinion needs to be moved further out of the housing.
- Too Deep (Flank-Heavy): Pinion needs to be moved further out.
- Too Shallow (Face-Heavy): Pinion needs to be moved deeper.
Note: Pinion depth (moving the pinion gear deeper or shallower into the housing) is a separate adjustment, usually done with shims behind the pinion head bearing, and should be set before carrier backlash. Adjusting pinion depth will change your backlash and require re-shimming the carrier.
Typical Backlash Specifications
Backlash specifications vary depending on the differential type, manufacturer, and intended use. Always consult your vehicle's service manual for exact figures.
| Differential Type | Backlash Range (inches) | Backlash Range (mm) |
|---|---|---|
| Standard Passenger Car | 0.006 - 0.010 | 0.15 - 0.25 |
| Heavy-Duty Truck/SUV | 0.008 - 0.012 | 0.20 - 0.30 |
| Performance/Racing | 0.005 - 0.008 | 0.13 - 0.20 |
| Used Gears | 0.008 - 0.015 | 0.20 - 0.38 |
These values are general guidelines. Always refer to the manufacturer's specifications for your specific axle model.
7. Final Assembly
Once both the backlash and gear mesh pattern are correct:
- Final Torque: Torque all carrier bearing cap bolts, and any other re-installed components, to the specifications found in your service manual.
- Install Differential Cover: Clean the differential cover and housing sealing surfaces. Apply a new gasket or RTV sealant and reinstall the cover, torquing the bolts in a crisscross pattern.
- Refill with Gear Oil: Fill the differential with the recommended type and amount of gear oil. Add any necessary friction modifiers for limited-slip differentials.
- Test Drive: Take the vehicle for a test drive, listening for any unusual noises (whining, howling). Avoid heavy loads or aggressive driving for the first few hundred miles to allow the gears to properly break in.
Setting backlash is a precise procedure that requires patience and attention to detail. If you are unsure at any step, consulting a professional or a comprehensive workshop manual is highly recommended.
