Hi Colin;
I agree with what Miles is saying that your problem is most likely a
warn rocker arm shaft and not the housing. If the housing is warn where the
shaft rotates then other things are wrong with the housing too. Normally
the rocker shaft wears, which is supported by a shinny area on the warn
shaft, both ends not just one. And, it is on the top of the shaft and not
the lower side. Although I have seen rocker arms ground smooth (they go
flat right at the center of the riding surface to the cam see the attached
picture) which caused problems too. The "popups" Miles is referring to are
Lash Caps like the ones used on the Ford 289's with solid rockers. Tim
Mings H_mings@... has reground cams with lash caps that really puts
zip back in your motor.
But back to your problem, if it is the rockers, then you can have them
hard chromed and reground.
If still will not adjust then remove your housing and have the bottom
surfaced removing 3 thousands and use a thinner gasket to bring the rocker
arm closer to the valve stem. Unless there is an adjustable rocker arm
available we are forced to make things work.
Bill