I can't believe there's no mark on the piston or valve if they are making contact. If it hits hard enough to break a rocker, there should be some evidence of contact. I would bring a piston up to TDC and compress a valve until it contacts the piston or binds against the valve spring. Measure this valve deflection with a dial indicator. I would suspect the wrong valve springs (coil bind) or cheap rockers. If a piston hits a valve, I would think a pushrod would bend first.
Scott
