Several possible reasons. Pushrods could be too long, causing the slot in the rocker to hit the base of the pedestal. Too long a pushrod could also cause the valve to hit the top of the piston, the weak link is the rocker bolt. Check that you have the cam timed right, if your cam timing (at the timing gear) is off, the valves could hit the pistons.
What cam and pistons are you using? Have you had the block decked or heads resurfaced? Are you using 360 heads? The valve could be hitting the piston wall if you are.
I ran 0 deck flat top pistons with a .488 lift cam in a 304, and had to have the pistons fly cut because of clearance issues. I always test assemble and clay check EVERY engine I build for valve/piston clearance before final assembly.
Bob
tufcj
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