REFERENCE · LIVE
A3 Diff Gear Pattern Library
Mark the ring gear teeth with marking compound, rotate the pinion. The contact pattern shows whether pinion depth and backlash are set correctly. Six patterns cover every common setup outcome.
SETUP PROCEDURE
- Apply marking compound to several ring gear teeth on both drive and coast sides.
- Apply light drag to the ring gear with a brake or pry bar (simulates load).
- Rotate the pinion to roll the ring gear ~1 full turn each direction.
- Inspect contact patterns on drive side (forward) and coast side (reverse).
- Compare to ideal — adjust pinion depth (shim) or backlash (carrier preload).
IDEAL
Centered, Slightly Toward Toe
Contact patch centered along tooth length and width, with a slight lean toward the toe (inner edge). Quiet operation, long life.
ADJUSTMENT: None. Lock down preload.
FAULT
Heel Contact
Pattern sits on the outer (heel) edge of the tooth. Whine under load, can chip the heel.
ADJUSTMENT: Decrease backlash — move ring gear toward pinion.
FAULT
Toe Contact
Pattern sits on the inner (toe) edge. Whine on coast, can crack root of tooth at toe.
ADJUSTMENT: Increase backlash — move ring gear away from pinion.
FAULT
High (Face) Contact
Pattern rides high on the tooth face. Pinion too far away from ring gear (shallow). Howl on drive.
ADJUSTMENT: Add pinion depth shim (move pinion deeper into the ring).
FAULT
Low (Root) Contact
Pattern rides at the root. Pinion too deep into the ring gear. Premature wear, root cracking risk.
ADJUSTMENT: Remove pinion depth shim (move pinion away from ring).
FAULT
Diagonal Contact
Contact diagonals from toe-face on drive side to heel-root on coast side. Combination of both pinion depth and backlash off.
ADJUSTMENT: Correct pinion depth first, then backlash. Re-pattern after each change.
BAY-FLOOR RULES
Setup discipline
- Set pinion depth FIRST, backlash second. Backlash is dependent on depth.
- Re-pattern after EVERY adjustment — clean off old compound, re-mark, re-roll.
- Patterns on used gears never look perfect — match the wear pattern, not the ideal.
- Backlash spec is typically 0.005-0.010" on most light-vehicle differentials. Confirm at OEM spec.
- Heel + face = "too much backlash, too shallow pinion" — common after lazy setup.
- Toe + root = "too little backlash, too deep pinion" — quiets the drive side but kills the coast.