Cut out some emery paper the same diameter of your valves and superglue it to the top of the valve head. Install barrell and pistons and bring the pistons up to TDC. Slip valves into guides with the sandpaer on them, but do not fit springs. Install head and slightly tighten down. Get electric or air drill and put the protruding valve stem in the chuck and and slowly bear down on top of the piston with drill on a slowish speed to cut valve reliefs on pistons. Be careful it works surprisingly well and you don't wanna take too much out of the piston.