MG Car Repair: Rocker Shims, rocker shaft, shims

Question
Hello Barrie,
I am in the process of removing the cylinder head of my USA 1980 MGB LE.  When removing the rocker there were no shims (as my manuals say there should be) under the the 2 middle rocker brackets.  What is the purpose of these shims and should I purchase and install when I re-assemble.  What effects would not having these in place have?
Thanks.
Wayne

Answer
Hi Wayne.  The rocker shaft is clamped into the 4 rocker pillars and must not rotate.  These shims introduce a very slight bend into the shaft to aid the clamping and prevent the shaft from fretting in the pillars.  

For road-going engines, you should insert one .005" shim under each of the two center rocker pillars.  This will help to prevent excess wear by keeping the rocker shaft from moving back and forth.

They should NOT be used on racing engines, where low friction is more important than rate of wear.