Differential Fluid Change - Rear

This procedure drains and refills the rear differential fluid on a 1972 Ford F-100 with 302ci V8, replacing the lubricant to maintain proper gear and bearing lubrication.

Warnings

Tools required

Parts

• Rear differential cover gasket × 1 — Ford 9-inch or Ford 8-inch differential gasket depending on axle type
• RTV silicone gasket maker (alternative to paper gasket) × 1 — Use OEM specification or high-temp RTV

Fluids

• 80W-90 GL-5 gear oil — 3.5 qt

Preparation

1. Park vehicle on level ground and engage parking brake firmly
2. If additional clearance is needed, safely raise rear of vehicle and support on jack stands placed under frame rails
3. Verify rear axle type by inspecting differential cover (Ford 9-inch has 10 bolts, Ford 8-inch has 10 bolts but smaller cover)
4. Place drain pan beneath rear differential cover

Procedure

1. 1

   Remove differential cover bolts
   Using a 9/16" or 1/2" socket (depending on bolt head size), loosen and remove all 10 differential cover bolts. Start by loosening the upper bolts first, then remove lower bolts while leaving one upper bolt partially threaded to control cover removal.
2. 2

   Drain differential fluid
   Carefully pry the bottom of the differential cover away from the housing using a gasket scraper or large flathead screwdriver. Allow fluid to drain into the drain pan. Once fluid flow slows, remove the final upper bolt and remove the cover completely. Expect approximately 2 to 2.5 quarts of old fluid to drain, depending on axle type.
3. 3

   Clean differential cover and housing
   Use a wire brush and shop rags to clean the inside of the differential cover, removing any metal particles or debris. Thoroughly clean the differential housing mating surface using a gasket scraper to remove all old gasket material and RTV. Wipe clean with a shop rag. Inspect the inside of the housing for excessive metal shavings or damage to gears and bearings.
4. 4

   Prepare new gasket
   If using a paper gasket, place it on the differential cover ensuring all bolt holes align properly. If using RTV silicone, apply a continuous 1/8" bead around the perimeter of the cover mating surface, circling the inside of each bolt hole. Allow RTV to set for 5 minutes before installation per manufacturer instructions.
5. 5

   Install differential cover
   Position the differential cover with gasket onto the housing, aligning all bolt holes. Start all 10 cover bolts by hand to ensure proper alignment. Tighten bolts in a crisscross pattern (star pattern) to ensure even gasket compression. Final tighten to the factory-specified torque (typically approximately 15-20 ft-lb for these cover bolts) in the same crisscross sequence; verify the exact spec for your axle type.
   ⚠Over-tightening cover bolts can warp the cover and cause leaks. Under-tightening will result in immediate leakage.
6. 6

   Refill differential with gear oil
   The differential is filled through the cover opening. Fill with approximately 3.5 quarts of 80W-90 GL-5 gear oil (Ford 9-inch) or 3.0 quarts (Ford 8-inch), checking the factory service manual for exact capacity based on your specific axle. Remove the fill plug using a 1/2" or 9/16" wrench. Using a fluid pump or squeeze bottle, add 80W-90 GL-5 gear oil through the fill hole until oil reaches the bottom of the fill plug hole (oil should just begin to seep out). If this axle is equipped with a limited-slip (Traction-Lok) differential, add the specified friction modifier additive to prevent clutch chatter. Install and tighten the fill plug securely. Verified detail (This step references a fill plug method that is incorrect for Ford 9-inch and 8-inch differentials. These axles are filled by capacity through the cover opening, not to a plug level.): Using a fluid pump or squeeze bottle, add the specified amount of 80W-90 GL-5 gear oil through the cover opening before final installation. Refer to axle identification to determine correct capacity (typically 3.5 quarts for 9-inch, 3.0 quarts for 8-inch).
7. 7

   Check for leaks
   Wipe the differential cover area clean with a shop rag. If vehicle was raised, carefully lower it to the ground. Wipe the area clean and lower the vehicle. After a short road test, inspect the differential cover gasket area and fill plug for any signs of leakage. Inspect the differential cover gasket area and fill plug for any signs of leakage.

Reassembly

1. No additional reassembly required beyond the steps outlined above
2. If vehicle was raised on jack stands, ensure all tools are clear before lowering

Verification

• Inspect differential cover and fill plug for any fluid seepage after initial test drive
• Test drive vehicle for 5-10 miles and recheck for leaks with vehicle parked on level ground
• Listen for any unusual noise from rear axle during acceleration and deceleration
• Verify proper fluid level by removing fill plug again - fluid should be at bottom edge of fill hole

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