Diffs that go bang in the back

[Mike Wattam <101714.1343@domain.elided>]

Hi John

Sorry to hear of your problem.  Starting with the easiest things first;

Could be a wheel bearing breaking up - lift the wheels to see if you have
any play or noise coming from the rear hubs.  Also suspect problems inside
the brake drum such as a broken shoe return spring or hub fixing studs/nuts
pulled out of swinging arm casing and floating around in the brake drum.

This last problem is not unknown and can be very serious.  If you
experience knocking from the rear hub area, remove the brake drum(s) and
check the 6 fixing studs and nuts securing the brake backplate to the
swinging arm.  These can tend to pull out of the swinging arm, and if they
will not take the correct torque (11-14 lbf-ft), strip it all out and fit
Helicoils to the swinging arm.  When refitting the studs, NEVER wind them
in on the standard Nyloc nuts, use locknuts to ensure the studs are fully
engaged in the threads, then fit the correct Nyloc nuts afterwards.  The
usual reason for this failure is that the person before you (isn't it
always?) had them out at some point and didn't bother to put them back
properly, - that is he left them hanging in by a couple of threads only and
as they were probably well over-tightened they were bound to pull out
eventually!  Once one pulls out, the others will soon follow.  Just think,
you could end up with your rear wheel overtaking you.

There have been a number of reported instances of the diff nose piece
breaking on the tubular section just to the rear of the crossmember
mounting flanges - this could be your problem.  Also bolts may be
loose/missing from the nose piece mounting flange to crossmembers, or the
large outer rubber mounts may have broken so the crossmembers drop onto the
large steel washers - visible from the side of the car - there should be a
'finger' gap between the cross member and the large washers.

Also look for the rear mounting plate of the diff broken or falling out of
the fixing studs in the body.

Assuming all is in fact well on the outside of your diff, it sounds as
though something has got into the teeth of the diff gears and marked them
so they make a noise on deceleration.  What usually happens is that parts
of the differential assembly itself (cross plus 4 bevel gears and thrust
washers inside a cage) break up and send debris into the main casing,
messing up the gears and bearings.  The only way to ascertain this is to
take the diff out and remove the back cover.

If this is so, a recon diff is the order of the day.

A 2.5 MkII saloon manual diff has the same ratio (3.7:1) and will fit after
changing flanges etc., or a MkII 2.5 (injection) auto saloon/estate had
ratio 3.45:1 which will give your Stag long-legged gearing (and a vast
improvement in driveability and economy if it is automatic).

However, you should be aware that the 2.5 diffs have smaller gear teeth and
bearings, thus may not last too long behind an enthusiastically driven Stag
engine.

Sorry I have no knowledge of a repair shop in Sydney who can 'do' your diff
- lets hope the locals can advise you, but if you want a replacement diff
sent out from the UK, either go to Moss locally or contact Rimmer Bros in
the UK who ship to OZ very often.

Please let us know how you get on.

Mike Wattam
Triumph Stag Register