boy this has been beat to death before on here. Theres guys who have 10,000+ miles on the sled and it runs fine according to them. Its not that they run bad or are so down on power issue. Its a simple wear issue. A compression test does not tell you the whole story as to if the rings are worn, just merely it can build pressure, its how long can you hold that pressure as to the true condition of the rings. Worn rings can still show you a good compression number but still have blow by into the bottomend. Its this blow by in which slowly removes the engines ability to make peak power.
The rings are also what holds the piston stabil in the bore, when they wear they allow the piston to now rock back and forth this wears the skirts on the pistons. If allowed to go long enough they will eventually break and fall into the engine and its not pretty.
As a guy who likely builds a few more engines then the normal guy on here, I will tell you rings need to be replaced around 5000 miles if you want the engine to stay at peak output. They will certainly run for longer mileage but they will wear the skirts and the power is not what it was when the rings were in better shape. Its a slow gradual loss of power because the rings wear slowly, this is what makes the guy say feels fine to me runs fine, but yet it wont pull peak rpm or maybe the sled doesnt pull the same clutch weight working on the clutch,etc.
The only true test is a leak down test, a compression test is just a rapid build of pressue but with worn rings it will lose that pressure right into the crankcase below.
you can also see this plainly by looking at the piston and the bottoms of the cylinders, youll see the black staining from the blow by!
lastly, if the rings were gonna run 10000-12000+ unlimited miles we would have 2 stroke cars. Lightweight engine with great power output. Because thats the drawback with a 2 stroke, the piston is the heart of the 2 stroke engine and they do require regular maintence to maintain the original power output. Most people would not want to take thier car in and have new rings put in every 5000 miles to keep the same gas mpg,power,etc.
do a search this has been discussed a whole bunch before, make your decision after more reading.
