drivetrain

Differential Fluid Change

for 1970 AMC Rebel Machine 390ci V8 · RWD

Editorial review:Chris Hackleman — Master Technician · 20+ years · Jeff Moore — Master Lexus & Toyota Mechanic · 20+ years

Difficulty

Easy

Time

1.0 h

Tools

Steps

✓Expert-verified. Personally reviewed and approved by OLP's master technicians (Chris Hackleman & Jeff Moore — 20+ years each). Always follow the vehicle's factory service information and torque specs.

Drain and refill the rear differential with fresh gear oil to maintain proper lubrication and extend component life.

Warnings

⚠Ensure vehicle is completely cool before beginning work to avoid burns from hot differential fluid

⚠️Always use jack stands rated for vehicle weight. Never work under a vehicle supported only by a jack

ℹ️The 1970 AMC Rebel Machine uses the AMC Model 20 rear axle with a removable rear cover

Tools required

3/8" drive ratchetEssential

3/8" drive socket set (metric and SAE)Essential

Drain pan (minimum 3 quart capacity)Essential

Fluid pump or squeeze bottleEssential

Wire brush

Scraper or putty knifeEssential

Torque wrenchEssential

Vehicle jack and jack standsEssential

Wheel chocksEssential

Shop ragsEssential

Parts

Rear differential cover gasket × 1 — AMC Model 20 rear axle gasket
RTV silicone gasket maker (alternative to gasket) × 1 — High-temp RTV sealant

Fluids

85W-140 gear oil (GL-5 rated) — 2.5 qt

Preparation

Park vehicle on level surface and engage parking brake
Chock front wheels securely
Raise rear of vehicle and support with jack stands under rear axle housing or frame
Allow differential to cool completely if vehicle was recently driven
Position drain pan under rear differential cover

Procedure

1
Remove differential cover bolts
Using appropriate socket, loosen all differential cover bolts in a crisscross pattern. Remove all but the top two bolts completely. Loosen the top two bolts but leave them threaded in several turns to support the cover.
2
Drain differential fluid
Carefully pry the bottom of the differential cover away from the housing using a scraper or putty knife. Allow fluid to drain into the pan. Once draining slows, remove the remaining two bolts and fully remove the cover. Tip the cover to drain any remaining fluid.
⚠Fluid may gush out when cover seal is broken. Keep hands and face clear of drain area
3
Inspect ring and pinion gears
With the cover removed, visually inspect the ring and pinion gears for excessive wear, pitting, or metal debris. Check for proper gear contact pattern. Inspect the differential bearings for play or roughness.
ℹ️Metal shavings are normal on the magnetic drain plug, but large chunks indicate potential gear failure requiring further inspection
4
Clean differential cover and housing
Remove all old gasket material from both the differential cover and housing mating surfaces using a scraper. Clean with brake cleaner or degreaser. Wipe out the inside of the cover, paying special attention to the magnetic drain plug if equipped. Ensure mating surfaces are completely clean and dry.
5
Prepare new gasket or RTV
If using a pre-cut gasket, position it on the differential cover and verify proper alignment with bolt holes. If using RTV silicone, apply a continuous 1/8-inch bead around the inside of the bolt holes on the cover, circling each bolt hole. For RTV, allow to set for 5-10 minutes per manufacturer instructions.
6
Reinstall differential cover
Carefully position the cover with gasket or RTV onto the differential housing. Start all bolts by hand, ensuring proper thread engagement. Tighten bolts in a crisscross pattern gradually and evenly to 25-30 ft-lbs. Make multiple passes, increasing torque gradually to avoid warping the cover or squeezing out the gasket.
7
Fill differential with gear oil
Locate the fill plug on the front or side of the differential housing (approximately at axle centerline height). Remove the fill plug. Using a fluid pump or squeeze bottle, add 85W-140 GL-5 gear oil through the fill hole until fluid begins to seep out of the fill hole. The correct level is at the bottom of the fill plug hole.
ℹ️AMC Model 20 rear axle typically requires 2.25 to 2.5 quarts. Stop filling when fluid reaches fill plug hole
8
Reinstall fill plug
Clean the fill plug threads and install the plug. Tighten the fill plug securely to approximately 25-30 ft-lbs. Wipe away any spilled gear oil from the differential housing.
9
Check for leaks
Inspect around the differential cover and fill plug for any signs of leakage. If using RTV, allow full cure time per manufacturer instructions (typically 24 hours) before driving. Wipe the cover area clean to facilitate leak detection during verification.