Differential Fluid Change - Rear

Drain and refill the rear differential with fresh 75W-90 GL-5 gear oil to maintain proper lubrication of the ring and pinion gears.

Warnings

Tools required

Parts

• Rear differential cover gasket × 1 — Use OEM specification or Fel-Pro RDS 55017
• RTV silicone gasket maker (if not using pre-cut gasket) × 1 — Permatex 81160 or equivalent

Fluids

• 75W-90 GL-5 Gear Oil — 2 qt

Preparation

1. Park vehicle on level ground and set parking brake firmly
2. Raise rear of vehicle using floor jack at rear axle housing or differential pumpkin
3. Support vehicle securely on jack stands positioned under frame rails, ensuring rear axle can hang freely
4. Allow differential to cool if vehicle was recently driven
5. Position drain pan beneath rear differential cover

Procedure

1. 1

   Remove differential cover bolts
   Locate the rear differential cover. Using an appropriate drive ratchet, loosen the cover bolts in a criss-cross pattern, removing the top bolts last to prevent sudden fluid spillage. Remove all bolts and set aside. (Verify exact bolt count and cover configuration against 1966 Corvette factory service manual before proceeding.)
2. 2

   Remove differential cover and drain fluid
   Carefully pry the differential cover away from the housing using a gasket scraper at the bottom edge. Allow the gear oil to drain completely into the drain pan. Once fluid flow stops, fully remove the cover and set aside. Expect old gear oil to drain into the pan; verify the exact differential fluid capacity against the 1966 Corvette factory service manual.
3. 3

   Inspect differential components
   With the cover removed, visually inspect the ring and pinion gears for unusual wear, pitting, or damage. Check for metal shavings or debris in the old fluid (some fine metallic residue is normal). Look for loose or damaged bearings. Address any issues found before proceeding.
4. 4

   Clean differential cover and housing mating surface
   Use a gasket scraper and wire brush to thoroughly remove all old gasket material and RTV from both the differential cover and the housing mating surface. Ensure surfaces are completely clean and smooth with no remaining residue. Wipe clean with a shop rag.
5. 5

   Install new gasket or apply RTV
   If using a pre-cut gasket, position it on the differential cover. If using RTV, apply a continuous 1/8" bead around the inside perimeter of the bolt holes on the cover, ensuring the bead encircles all bolt holes. Allow RTV to set for 5 minutes if specified by manufacturer.
6. 6

   Reinstall differential cover
   Carefully align the differential cover with gasket onto the housing, ensuring the gasket remains properly positioned. Thread all 10 bolts by hand to prevent cross-threading. Tighten bolts in a criss-cross pattern gradually, working from center outward. Torque bolts evenly in two passes to the factory specification (verify torque value against the 1966 Corvette service manual).
7. 7

   Refill differential with gear oil
   Locate the fill plug on the differential housing (verify exact location against the factory service manual). Remove the fill plug using an appropriate drive ratchet. Using a fluid pump or squeeze bottle, add the factory-specified GL-5 hypoid gear oil through the fill hole until oil begins to seep out of the fill hole. If the differential is a Positraction limited-slip unit, add the required limited-slip friction modifier additive. The differential is full when fluid reaches the bottom of the fill hole threads.
8. 8

   Reinstall fill plug
   Clean the fill plug threads and inspect the plug seal or gasket for damage. Replace if necessary. Thread the fill plug in by hand, then tighten securely to the factory specification (verify torque value against the 1966 Corvette service manual). Wipe away any excess oil from the housing exterior.

Reassembly

1. Verify all bolts are properly torqued and no leaks are present at the cover
2. Lower vehicle from jack stands using floor jack
3. Dispose of used gear oil at an appropriate recycling facility

Verification

• Start vehicle and allow to idle briefly, then check for leaks around differential cover and fill plug
• Test drive vehicle at low speed for 5-10 minutes, listening for unusual whining or grinding noises from the rear differential
• Re-inspect cover and fill plug for leaks after test drive

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