Why and When to do top end work
 

Our 09 RMZ250 is definately due for valve adjustment, but I'm wondering if the piston/ring needs attention too. The bike has had 2 seasons of hard use and some racing on it, but doesn't show any signs of problems.

Two different expert riders have said the valves seem tight, and that makes sense due to the use.

My familiarity with 4-strokes is mostly Honda trail bikes, and they smoke once the rings start to go. Someone on another forum said that performance 4-strokes will "blow-up" long before they start to smoke, but didn't respond when I asked "what exactly blows up that I need to check or replace?"

Can anyone give me a little info on why and when a 4-stroke top end should be torn down and re-done....and maybe what parts are standard to replace at that time?

Engine is apart. Compression was low (31 psi) due to slight cylinder scratching and worn rings. Valves are fine and still within spec. No end play or side play in crank. Rod bearings check fine too. Not bad for between 200 and 300 hours.

The old cylinder could be re-used as is, but as it's apart, it's probably best to replace it. Going with a new cylinder vs replating, as it's only $345 and re-plating is about $300 plus shipping back and forth. New piston and rings too.

As far as fatigue goes and crank/rod replacement "just in case", as I can tell, metal has to either bend past the design limits or a crack has to start. Apart from NDTing, the only alternative would be blanket replacement. I'm not prepared to do that, even though I may regret it later, as where do you draw the line?