I have used the OS bubbleless clunk in my pattern airplanes and consider them a neccesity. Now all of my sport planes have regular clunks and i have never used a bubbleless clunk in them. The OS clunk is about 15 bucks. what i usually do in my sport planes is take a dremel cut off wheel and grind another slot on the pick up. so it is an X not just the one slot.

Dennis