ORIGINAL: mikenlapaz
Could you elaborate on or direct me to a reading source for a guideline know whether a bore or rings are worn out.
I have no idea where or how that measurement in quote 1 would be made. I suspect that it requires more than a set of dial caliper. They would not reach into the bore to measure where the ring(s) would contact.
You need to measure the piston and bore separately; the former using an outside micrometer and the latter using a bore gauge (preferable) or an inside micrometer. Then you have to subtract and divide by 2. For the bore, a total of 6 readings are to be taken, 3 of them at three points along the height of the bore (area of ring travel only), and the other three perpendicular to the first 3 at the same heights. Then average out the readings. Piston is to be measured at maximum diameter (usually perpendicular to the wrist pin and at the bottom).
The thumb rule I use for clearance is 0.03 - 0.04 mm per 25 mm bore. For ring end gap the rule is 0.1 mm per 25 mm bore.