Changing the rear-axle (differential) oil on a Triumph Stag

Why, and how often

Renew every few years and top up if low. The Stag's differential is a hypoid unit — the ring and pinion mesh with a sliding action that ordinary gear oil cannot protect. An EP (extreme-pressure) hypoid oil is required.

Steps

1. 01. Warm the car and level it

   Warm the car with a short drive, then chock and raise it so it sits level.

2. 02. Filler plug first, then drain

   Find the filler/level plug FIRST (if it won't undo, don't drain). If a drain plug is fitted, drain fully, clean it, refit with a sound washer.

3. 03. Fill to the level hole

   Fill through the level hole with the correct EP hypoid oil until it just weeps from the hole. Don't overfill — excess can be forced past the output seals.

4. 04. Refit and leak-check

   Refit the plug; check for leaks after a run.