Planning on taking your motor apart? Got it apart already?

• 1) Valve Guides: You put a valve in the guide and see if you can get any "wiggle" - feel any side clearance. This with a dry valve and guide, once you've cleaned it all up. If you feel clearance, you can then use a micrometer to check and see if the valve is worn. (But it's usually the guide.)
• 2) Valves: I have a thing with a suction cup on a stick and grinding compound. The procedure of "lapping" the valve and seat has value only in checking to see if Ican get a proper sealing surface (a "ring" about 2mm wide on both their surfaces. If Ican, I know Idon't need to have them refaced on a valve grinding machine, and having a valve refacing machine recut the seats. If the valve or seat is too far gone, the valve will sit too far down into the seat, and either or both will have to be replaced. The machine shop with the refacing gear will help you make this decision. (nb: once the refacing is done, "lapping" with grinding compound is totally unecessary - there is an old wive's tale still in vogue in some circles which says this will improve things, but it won't - conditions are totally different when the engine is hot and running fast than they were when you lapped the seats cold, so there won't be any improvement.)
• 3) Rings: You take them off the piston, and put them in the cylinder squarely, at top, center, and bottom of stroke. Using a feeler gauge, measure the "end gap" - the distance between the ring ends. If it's over the specs in the manual, you replace the rings. If new rings show too much gap, you know your cylinder is too far oversize. (Variations in end gap in different parts of the cylinder will also give you an idea of cylinder wear - remember that gap differences will be 3 times diameter diffs. Well, 22/7)
• 4) Piston clearance: You put the piston in the cylinder (both dry and clean) and using a wire feeler gauge, check for skirt clearance. (at right angles to the piston pin) If it's more than the manual says for piston clearance, you get a new piston and try again. If it's still too much, you have the cylinder rebored to the size of the new oversize piston you exchanged for the first new one you just bought. Never having had a good set of wire gauges, I've always used a narrow blade gauge.
• 5) Big end: Check for side clearance of the top of the rod at different places - top dead center, middle, and bdc. See manual for decision-making here. If the side play is getting up there, check for end play - if you can feel any up-and-down clearance at all, it's time for disassembling the crankcase and having a new big end pressed into your crankshaft. You have this done at a knowledgable motorcycle repair shop.
• Other Stuff: Cam bushings, cam followers, oil pump drive, all these and more can be decided upon by the same basic methods. Check when dry and clean for sloppy fit axially, and check the manual to see if end play is a consideration - although you can often figure it out for yourself (would it make a difference if this thing moved end to end? Is there anything here that is designed to prevent end play, but is excessively worn?

These are the basic considerations to use in deciding what to replace when you have your motor apart. This is only a guideline; you should of course have a copy of the service manual before you attempt to do any kind of rebuild on any kind of engine, and read this carefully before embarking on your maiden voyage into wrenchland. Happy monkeywrenching!