From Ross :
Free drop is the how far the valve moves from when it is on the seat until it intersects the plain of the head surface.
Example: Set the head on a flat surface and measure a bare valve from its seat,(how much stem is sticking out of the guide) let it drop until it touches the surface table and measure the stem again.


Trying to find out what cam size I can use without having piston/valve clearance issues before I order the cam. Spoke with one cam manufacturer preferred by my engine builder and they said they wouldn't predict it. Spoke with Ross who's pistons are in this 451 build and they wanted the 'free drop' of the intake and exhaust valves. I don't have my heads handy right now so can't make the measurements myself.

Sounds like I should check each of my heads if the machining has a lot of variance head to head. But I guess if there's a typical number for -1's (like .250 Andy mentions), then I could rule out certain size cams if it show's obvious clearance issues.