The quick & dirty way to check for piston to valve clearance is to get the piston to the top of the stroke on overlap (exhaust closing/intake opening) and pry the rocker arm down until the valve touches the piston. A dial indicator on the spring retainer will show how much the head can be milled without piston/valve contact. I like to keep at least 1/16" of safety margin on street engines, or a bit more, maybe .090" if the engine will be revved close to the valve float point on occasion.
Jerry

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