The MGA With An Attitude
BMC # 12H3960 - SHIM, pedestal tensioning, centre pedestals only, 2 req'd.
Moss Motors USA # 460-255 - SHIM, from 18GB engines on, 2 req'd.

At 11:41 PM 6/1/04 -0400, Tony Cox wrote:
>"Can you explain the purpose of shims that I have found under the rocker shaft brackets. My MGA has one on the second bracket in from the back of the engine. I have also seen them on MGB engines that I have taken apart but can find no reference to them in parts books or service manuals."
Shim under rocker pedestal
The Moss MGB parts catalog shows them, 2 per engine starting with the GB engine (first 5-main bearing engine). It was also recommended by the factory in a service memorandum that the shims should be installed in earlier engines whenever the rocker shaft might be removed for servicing.

The shims are 0.010-inch thick and should be installed under #2 and #3 pedestals. This applies a preload (and very slight bend) to the rocker shaft, taking out any clearance around the rocker shaft and holding it securely in place so it cannot wiggle (which could make for a little wear on the shaft or bores of the pedestals).

I have heard at least one experienced mechanic say that he has noticed visible wear on the rocker shaft in some high mileage engines without the shims, but that such minor wear would most likely have negligible effect on the running of the engine. This might be more important for the MGA using original type aluminum rocker pedestals than in the MGB using iron pedestals.

I have never used the shims on my MGA, and likely never will, never having any such problem there. However, I have long since converted all of my MGA engines to iron pedestals transplanted from an MGB engine.

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