I run my '18 with OE pistons and the F3 with, er F3 pistons. If you did want to run the F3 pistons though you could run the long skirt ones which do help ring life.

Either long or short skirt F3 pistons will require the barrel to be machined though to get the best from them. However thats the same with OE pistons if you want to be picky.

The single rings dont last too long though and I'm replacing mine, as Andy says, around every 200 miles. You can feel the engine running freer and freer as they wear. The instant you put a set of new rings in it it feels like you've got another 5hp back.

If it was a track bike thne its easy to whip the cylinders off and change, but if its a road bike I woudl be sticking with the twin ring OE ones.
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If I have to take the carbs off once more...