I had considered doing just the rings, but realistically most of the effort lies in cleaning the bike up enough to get to the piston. Pulling parts off is easy! Making sure there is no dirt around to go where it shouldn't takes a bit more time and effort.
I figure for the effort its easier to just do the whole slug. Many times you do start to see wear on the skirts, as well as the ring end opening up. The rings do wear the ringlands to a certain extent, and the wear on the skirts allows the piston to rock in the barrel more than it would otherwise which likely increases were on the nikasil. For all those reasons I simply replace the piston - it's not that much money for the time and effort you spend doing the job (inc cost of gaskets, cleaning products, etc).
I have found the stock vertex pistons start to get noisy around 70hrs. Happened on both my bikes. On the newer one, I did the first piston at 65hrs and everything looked pretty good wear wise. I have some pictures somewhere and could check my notes on wear tolerances, but do recall thinking they weren't too bad.. but I could hear the piston rattle a bit on decel.
I was going to stretch this interval out but the bike was really feeling quite down on power. Very notable drop in bottom and mid. The comp reading confirmed my thoughts. The wear pattern and ring end tells the story.
On my other engine I put 110hrs on a slug and rings, and the ring end gap was bigger, but I don't recall the power dropping off like it has this time.
It may also be a case that I am riding the bike in more challenging terrain, at larger throttle inputs and simply wearing things out in shorter time frames.
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