The rear axle and differential

• Never work under a car held only by a jack. Use axle stands on firm, level ground and chock the wheels that stay on the ground.
• The differential is heavy. Support it on a jack before undoing its mountings, and have a helper when lowering it.
• If the car feels loose at the rear, or the handbrake cable suddenly tightens, stop driving and have the rear subframe checked (see below).

Rimmer Bros list the differential mountings, a mounting plate and the rear cover. The club's technical reprints advise that hardened or worn mounting rubbers transmit noise, and that a loose differential cross member at the nose mounting lets the differential nose down and creates noise.

The studs or pins that locate the differential mountings in the body are a recurring owner topic. Owners report pins coming loose and turning with the nut, and that a lasting repair can mean opening the box section or the floor above to weld them properly. That is owner advice: the archive has not found a factory procedure for it, and it is structural work for a competent welder.

Suppliers describe the front pinion oil seal as a common weak spot. LDpart notes that the leaking oil can wash the grease from the bearing behind it, so do not ignore a drip from the front of the differential.

Before renewing the seal, check two things the club's technical reprints point to. First the breather, a hole with a split pin loosely fitted inside, on the offside top face of the rear casing: if it is blocked, oil will be forced out of the front seal. Second, that the differential has not been overfilled.

The reprints also note that a new seal may leak if its lip rides on a groove worn in the shaft by the old one, and describe either having the groove dressed by an engineer or seating the new seal so its lip runs on unworn metal. The archive treats renewing this seal as specialist work, because the differential generally has to come down to reach it.

The club's technical reprints give a checklist before condemning a whining differential:

1. Are the rubber mountings sound, with no metal to metal contact?
2. Is the cross member at the differential nose tight?
3. Are the tyres sound? Some tyres whine; swapping in known good ones settles it.
4. Wheel bearings can whine too, but the noise is usually speed sensitive from low speed and seems to fade at higher speeds. Check for play at the rear wheels.
5. Are the propshaft and driveshaft joints sound and free?
6. On an overdrive car, does the noise change with overdrive in or out?
7. Do the differential output flanges have play?
8. Is there oil in the differential?

Note whether the noise changes between drive and the overrun, or follows road speed only. The rear whine and wheel bearing rumble fault finder entries give the tests. The reprints add that all differentials whine to some degree, that a reconditioned unit can be noisier than the original, and that a new one needs a few hundred miles to settle.

The rear subframe is mounted to the body through rubber mountings, listed by suppliers in standard rubber and in polyurethane. Worn mountings give clonks on and off the throttle and contribute to the rear end steering effect covered in curing the Triumph twitch.

A North American Stag club technical article warns about the subframe cracking at its outer end, where the support for the body mounting is welded to the box member. It describes a fatigue failure that lets the trailing arm move rearwards, pulling the handbrake cable tight, and recommends cleaning the grease off that area and checking it for hairline cracks with an inspection lamp. A cracked subframe is a stop driving fault.

The archive's rear axle oil page gives an EP hypoid oil, EP80W-90 or straight EP90, with a capacity of 2 pints (1.14 litres) from dry, filled through the level plug until it just weeps.

There is a disagreement to note. The servicing page says GL-5 is fine. A Stag Owners Club technical panel answer in the club's reprints advises GL4, and warns against GL5 because its additives were found to attack the copper thrust washers. The archive has not yet settled which is right and shows both.

• A factory procedure or torque figures for the differential mountings or pins.
• A pinion nut torque or preload figure; use the workshop manual.
• A factory procedure for repairing loose mounting pins in the body.
• A definitive answer on GL4 versus GL5 for the Stag differential.