If you do not have good instruments don't bother trying to measure the bore/piston to determine a clearance. If you run good oil and keep a clean filter, the ring wear will not be a factor. I run 75 - 100 hrs in a 250, compresion is still over 180 PSI, piston looks great through the ports with little to no blow by. I'm sure if I tore it down to measure it it would be in spec. I also run 36:1 premix. The next issue is fatigue of the aluminum. This is the limit you will eventually hit. If there is wear, increased clearance, and piston slap, this will be accelerated and the skirt can fail. For all the time it takes to tear down, clean, and measure parts, I'll do the complete top end swap at 75-100 hrs. I don't bother doing rings alone.
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