I'd invest the $30 +/- dollars in gaskets to pull the head and have a look at the piston, valve and valve guides. A set of valve guides and seals isn't that expensive. It sounds like either you've got a problem inside the head with the guides or the valve, or the valve is hitting the piston. If its hitting the piston, you should notice it.

In your other post you mentioned that the engine runs fine until the rocker breaks. Is it all old ones breaking and the new ones are doing fine? If that is the case I'd say metal fatigue is still a possible candidate and you should replace anything you havn't replaced yet.

Another thought, how does the shaft the rockers ride on look? Any chance the rocker is seizing to the shaft when it heats up and then breaking?

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi