I find that making Pistons is a bit of a challenge and also a time consuming pain in the neck.
But, once I got into the mind set of making them it made sense to carry on and make all the ones I needed in one long lathe session.
The Mercury piston came about by accident due to the firing cycle of my Original 35, the Mercury piston is being made to a 70mm stroke, but I've left the option to long stroke it to 80mm with this piston if needed --- I'd just have to machine 10mm off the back of the top hat.
All the best Mick