when claying? valve spring?
the clay method is the best methods imo. it is physical evidence from the valve as too how close it comes to the piston during the entire rotation of the cam. the springs have no bearing whatsoever on PTV clearance. unless, that is, the wrong springs are used to begin with and you get coil bind. but thats another problem all together. you will need a solid lifter, or a, modified to be solid, hydraulic. if using an adjustable p-rod, put the lifter on its base circle and adjust the p-rod to zero lash. you skimp in any way on this check, and you will be hating yourself when you get the engine together. i prefer to do my check with completely assembled heads, with the springs i plan to use, and no head gasket.
