I did a tear-down and refurbish on my K75, putting a new (used) engine in to replace a tired one a couple summers ago.
Logic for the teardown is that I wanted new gaskets and was worried (correctly) about crap in the cylinders from storage. Also wanted to inspect to see that it is at factory spec.
With a K bike, you really aren't doing a rebuild when you do an overhaul, especially short of a ludicrous mileage. The cylinders are not able to be overhauled (absent serious cost/effort to re-coat with nikasil and custom pistons/rings), and rarely need it: the Nikasil is insanely hard. Pistons and rings wear, the bores do not. At 100k on my previous engine, with some serious abuse (questionable maintenance history, 4000 HARD miles with a stuck open thermostat, probably more before that.), you could feel a slight ridge at the top of the cylinder, and there was noticeable piston/cylinder clearance. However, you could still see some crosshatching.
The ~35k replacement engine was all at factory spec: I checked it all. You could barely tell it had been run, once the oil and such was cleaned off. Cylinders were crosshatched still. All you need to do is scrub them with green scotchbrite to get rid of some glazing.
The one part of the engine that does occasionally need (and is able to be) rebuilt is the head. The exhaust valves recede, and eventually you use up the available adjustment range. The seats are installed with a range of acceptable depths. If they are on the tighter side from the factory, there will be less life expected. There's a guy with (last I read) over 800k on a K75. He needed a replacement (used) head at 320k, and at 500K had a rebuild. At 500k, he had the head built for the full clearance to be availiable.
