Re: M21 JP pistons - need opinions based on experience!!
I have a JP piston in my rigid 350 AJS trials bike. I fitted it when I rebuilt the engine in about 2011, but it was very heavy as bought. We removed quite a lot of material from inside the crown to get the weight down. The whole skirt of the piston was coated with a baked on teflon type coating. The barrel was bored to give about 3 thou of clearance. It has been faultless ever since and has done lots of hard work since.
