Quick guide to those pistons...
BSA used dished and flat top pistons, this alters the compression ratio for the various model variants.
Ring groove...top and second are 1/16" oil ring is 1/8". Most brands of ring come ready to fit, ring gaps are a measure of bore wear by eye in my backyard...I've never agonised over this and had no problems.
They do indeed sound like Cord Rings, back in the day and even now if you can find them, they were a good cheaper alternative to a rebore and new pistons. The first and second rings are two spring steel segments which are fitted to bear against each other at the bore, and against the top and bottom of the ring groove at the inner periphery. Pressing the ring segment on a flat surface makes one side lift up. Call this "Cup Up" so to fit them, put the upper ring in the groove cup down, lower ring of the pair goes cup up. Plenty on the web.
Pistons you have I reckon are "Plus 60's", in other words to fit a standard 70mm bore, opened up by 60 thousands of an inch, or 1.5mm. If the bore measures 71.5mm, that's what you have. Somewhere there will be a brand name and casting numbers which will help identify them. The rings will cope well with a worn bore, so if there is no apparent damage, give everything a good clean and continue to use them, as Reg suggests. Pistons are expensive, so don't just dump them if and when, someone else will find a use for them.
Swarfy.
