Service the rear differential on a 1966 Corvette by draining old gear oil, inspecting the cover and gears, replacing the gasket, and refilling with fresh GL-5 gear oil.

Warnings

Tools required

Parts

• Rear differential cover gasket × 1 — Use OEM specification or equivalent for GM 10-bolt or 12-bolt differential
• RTV silicone sealant (if gasket not used) × 1 — Permatex 81160 or equivalent high-temp RTV

Fluids

• 75W-90 GL-5 Gear Oil — 2 qt

Preparation

1. Park vehicle on level ground and set parking brake firmly
2. Chock front wheels securely
3. Raise rear of vehicle using floor jack at rear differential housing or frame rails
4. Support vehicle on jack stands placed at manufacturer-recommended rear frame points
5. Allow differential to cool if vehicle was recently operated
6. Verify adequate clearance to access differential cover from underneath

Procedure

1. 1

   Position drain pan and locate fasteners
   Slide drain pan under the rear differential carrier. The 1966 Corvette IRS uses a cast-iron center carrier without a removable stamped-steel cover. Locate the drain plug at the bottom of the carrier and the fill plug on the side; service is performed through these plugs, not by removing a cover.
2. 2

   Loosen all cover bolts partially
   Using a 1/2" or 9/16" socket (depending on bolt head size), loosen all cover bolts approximately 3-4 turns but do not remove them yet. This allows you to control the oil drainage in the next step.
3. 3

   Break cover seal and drain oil
   Loosen the bottom two or three cover bolts completely while leaving the top bolts partially engaged. Carefully pry the bottom of the cover away from the housing using a scraper to break the seal. Allow gear oil to drain into the pan. Once flow slows, remove remaining bolts and carefully remove the cover, being prepared for additional oil to spill out.
4. 4

   Inspect differential internals
   With the cover removed, visually inspect the ring gear teeth for excessive wear, pitting, or damage. Check for metal shavings or debris in the drained oil and on the magnetic drain plug (if equipped). Look for any signs of bearing failure or gear tooth breakage. Minor metallic residue is normal, but large chunks or excessive material indicates potential failure.
5. 5

   Clean cover and housing mating surfaces
   Thoroughly scrape all old gasket material and RTV sealant from both the differential cover and the housing mating surface using a gasket scraper. Use a wire brush to remove stubborn residue. Clean both surfaces with brake cleaner and wipe dry with clean rags. Ensure mating surfaces are completely clean and smooth for proper seal.
6. 6

   Prepare new gasket or apply RTV sealant
   If using a replacement gasket, position it on the cover and verify proper alignment with all bolt holes. If using RTV sealant instead, apply a continuous 1/8" bead around the inside perimeter of the bolt holes on the cover, ensuring the bead encircles all bolt holes without gaps. Allow RTV to set for 5-10 minutes per manufacturer instructions if specified.
7. 7

   Install differential cover
   Carefully position the cover with gasket or RTV onto the differential housing, aligning all bolt holes. Start all 10 cover bolts by hand to ensure proper thread engagement before tightening any of them.
8. 8

   Torque cover bolts in sequence
   This vehicle's IRS differential has no removable service cover, so there are no cover bolts to torque. After draining and refilling, torque the drain and fill plugs to the factory specification (verify against the GM service manual for the 1966 Corvette).
9. 9

   Refill differential with gear oil
   Locate the fill plug on the passenger side of the differential housing (typically a square-headed plug or bolt). Remove the fill plug using a 3/8" ratchet or appropriate wrench. Using a fluid pump or squeeze bottle, add GL-5 80W-90 gear oil through the fill hole until oil begins to seep out of the fill hole. If the unit is a Positraction (limited-slip) differential, add the required GM limited-slip friction modifier additive. The proper level is at the bottom of the fill hole threads. Install and tighten the fill plug securely.
10. 10

    Lower vehicle and test drive
    Remove jack stands and carefully lower the vehicle to the ground. Test drive the vehicle for 5-10 miles, including various speeds and turns. Listen for any abnormal whining, grinding, or howling noises from the rear end that might indicate installation issues or pre-existing gear problems.

Reassembly

1. If RTV sealant was used, allow 24 hours for full cure before subjecting differential to heavy loads or high-speed operation
2. Dispose of old gear oil at an approved recycling center or automotive parts store

Verification

• Inspect differential cover for any signs of oil seepage after test drive and again after 50-100 miles
• Verify fill plug is secure and not leaking
• Confirm smooth operation with no abnormal rear end noise during acceleration, deceleration, and turning
• Check gear oil level at fill plug after first 100 miles to ensure proper fill level is maintained

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