For all MGA, MG ZA and ZB Magnette (and sister cars), and pre-1968 MGB with banjo style rear axle, side thrust washers for differential gears were originally bronze. These have a very long life, possibly lasting up to 250,000 miles. For 1968 and later MGB (and all earlier MGB GT) with tube type (Salisbury) rear axle, the differential side thrust washers were fiber material. These parts are physically interchangeable, but the fiber washers have much shorter wear life, perhaps 80,000 miles before needing to be replaced.

In recent years most parts suppliers have been selling the (cheaper) fiber washers (shown at top of this picture) as replacement parts for cars that originally had bronze washers, and the bronze parts have been unavailable for a long time. I take this to be a big mistake, as someone should be supplying bronze washers for all of these cars including the later models which originally had fiber washers.

Finally someone has taken the initiative to reproduce these parts in bronze. Paul Middleton <Paul.Middleton@dps.texas.gov> (email failed 4 May 2020) in Texas, USA, has produced a limited quantity of these parts. They are currently (March 2013) listed on eBay, available for immediate delivery. For as long as the ad may last see here: www.ebay.com/itm/261202484997. If there is sufficient demand I am sure more will be produced as required.

Addendum July 30,2013:
Here we have documentation of a known life span (one case) of the late model MGB fiber thrust washers.

On 7/27/2013, Russell Goebel in Australia wrote:
"After purchasing the car in 2001 the left rear axle separated from the housing (while driving) after some so called restoration "expert" forgot to install the lock tabs on the rear axle correctly. The diff housing was damaged so the innards were swapped out and the diff set up at a specialist engineering works. Since that time the car has traveled about 50,000 miles.
There seems to be some slack in the diff. Handbrake on and in neutral there is approximately 3/8 inch (10mm) of movement at the input end. This seems to be too much freeplay".

On 7/29/2013, Russell Goebel wrote:
"I have sourced some brass washers from the local MG spare parts dealer, has been in business on the same site same owner since the early 60's. These are 0.005 os (actual 0.040-in). The standard washers he had were fibre and 0.035-in".

On 7/30/2013, Russell Goebel wrote:
"Got the new washers in and the diff re-assembled. It went together with about 0.002 clearance between the brass washer and the gear.
The differential had fibre washers. One was okay the second was almost completely worn away. The diff was re-built 50,000 miles ago by an engineering shop. So assuming they put fibre washers in at the time that confirms the 50,000 service life of fibre. Attached a picture of the old fibre washers".

Addendum, March 17, 2021:
There is another source for the bronze thrust washers from Rimmer Bros, UK, part no: ATB7072040 THRUST WASHER +0.005" OVERSIZE (0.040"). 2.45£ ($3.41) to date. these are oversize and will not fit without some re-machining.