stihl only
ArboristSite Operative
I asked the same question a while back and the replies boiled down to:
1 ring – tighter tolerances,
2 rings – looser tolerances, thus two rings to stabilize the piston,
and I forget which one was more efficient at heat transfer.
Well the skirt is what makes the piston stable and with the rings right next to each other, an extra one wouldn't make it a bit more stable.
I'm not a book learned engineer but in my mind, two rings=twice the surface area to transfer heat.