You gotta see this ......

I should have been more clear regarding the heads, I confused the issue a bit by bringing up the 340 heads. I agree that's a piston to head issue with the chamber, the valve height doesn't change.

But even with the valves in the same spot regardless of the head, the valve to piston clearance is more of a consideration on the early 340's because of the piston height.

Of course, we don't know what pistons or cam was used yet, or if the clearance was checked. I just thought it was worth mentioning, it's a pretty spectacular failure for just stiff springs. Not impossible, but I'd check a few more things before just replacing the rockers.