GEC52,
I tend to agree with Tiny...it seems the push rod would always bend before a rocker would break. If a valve seized in a guide tight enough to break a rocker arm, it doesn't seem like it would "unseize" by itself.
I would focus on the rocker arms themselves. Is the assembly getting the proper amount of oil? Could the engine have overheated at some point in its life and damaged the integrity of the rocker arms?
It sure seems like you are doing everything right in the adjustment department. When you replace the rocker arms that have broken, do they break again! Short of bottoming out a spring or having the valve kiss a piston, It would appear that the cause of the failure would be the rocker itself.
Stuart
