S1 Aprilia and ALL Augustas are on inch Lockheed , S2 Aprilia , appia etc (post war) are on sabif , never had a problem with pistons if I'm honest , must be lucky , all the genuine pistons are brass or steel , replacement again usually post war are alloy , new pistons is not something I'm going to get involved with at this stage as I have too much on atm , I think if you were going to do it stainless would be the answer not brass , brass is good but still corrodes