A wise man once told me that original Black Ariel pistons had a habit of expanding, but not contracting. His advice if it nipped up was to clean up the high spots on the piston (file not emery) and try again.
Swindon Branch Secretary
Ariels - 1913 TT Model, 20 Roadster, 28 Model D, 30 Model B, 38 VH, 52 VH
The Offroad Fleet - Bultaco Pursang, Beta Rev 3