In the MGA Twin Cam Workshop Manual on fitting the oil pump drive spindle in section A.32 (Issue 2) or A.31 (Issue 3) it says to fit the "thrust washer in position". However it does not say where to fit it, above the gear or below the lower flange. It needs to be understood that all helical gears produce an end thrust on shafts. On the pushrod engine the spindle thrust is downwards and the thrust is taken by the spindle flange against the oil pump. On the Twin Cam, even though the spindle turns in the same rotational direction as the pushrod one, the counter clockwise rotation of the half speed shaft and the resultant different angles of the teeth on the two drive gears, the thrust is upwards. The thrust washer must be fitted above the spindle gear.

Images left to right:
  Twin Cam Service Parts List plate AB, no thrust washer shown.
  Twin Cam Service Parts List plate AE, no drive gear shown.
  Twin Cam drive gear with thrust washer correctly positioned on top.
The lack of information is made worse because some people may refer to the factory Service Parts List which shows, incorrectly, the thrust washer as item 17 on the page with the oil pump, facing page A.10 (Edition 2) or facing page A.13 (Edition 3). This gives the false impression that the washer could go above the oil pump and below the spindle flange. The washer should be shown with the spindle, facing page A.3 (Edition 2) or facing page A.6 Edition 3). Using high magnification the washer can be seen above the gear in the Workshop Manual transverse section drawing of the engine. If not fitted correctly the gears may not be fully meshed.

The washer should be hardened and ground steel with an I.D. of 0.496" and an O.D. of 0.748. The thickness is as required to give an end float of approximately 0.010" to 0.020" and is normally approximately 0.090" thick.

See Part Numbers article AEH460 for more information on the thrust washer and other parts of the oil punmp assembly.