It would have to be WAY down the valve guide, near the full open dimension. Fortunately, the intake valves are the only ones that get close to the pistons, and they're almost at right angles to the piston top. The exhaust valves are way up in a recess in the head, and it's impossible to get piston contact. If you're turning the engine by hand and there's piston/valve contact, the piston just tries to push the valve closed without bending it. At running speed it's a whole different kettle of fish!
You can deal with sticky valves by getting the engine turned several degrees past top dead center where there's no possibility of valves and pistons getting together, soaking the stems with penetrating oil, and repeatedly tapping the rocker arm tips with a dead blow mallet- - - - -bouncing the valves up and down on the seats. That has a tendency to dislodge any rust or crud buildup. Really stubborn sticky valves require pulling the head and a detailed cleanup, however.
Jerry
