So question since we're on the topic of valves.
How do you know when your valves need changing other than knowing by the valve adjustments.
2. When installing new valves, is it ok to "lap/seat" these Ti valves? If not qhats the correct way
3. Grayracer said he has his seats refinished, what does that mean?..How to know when seats need to be changed?
4.What about valve springs?
5. What about valve guides?
Assuming we are still talking about a YZ450 (or a 426 with Ti valves:
1) If you have had the bike a while, you've probably had it during at time when it has gone months or longer without needing any adjustment to valve clearances at all. As it wears, one or more will need to be adjusted one shim size smaller. When it comes time to shim that same valve another size smaller, it's time to replace the valve. Otherwise, physical inspection will let you see the wear condition, and a leak down test will tell you whether and how much they may be leaking.
2) Absolutely, unequivocally NOT. Not under any circumstance ever. Lapping with abrasive pastes will attack the hard coat as well as the seat, and the hard coat that the valve depends on is less than .001" thick. Even with un coated steel valves, the seat must be precision refinished BEFORE any lapping is done. Otherwise, the wear pattern of the used seat will be partially transferred to the valve face in the process.
3) Valve seats are properly refinished in three main ways: Precision cutters by hand, or better, precision grinding with specialized equipment, or best, recut in a specialized machine resembling a vertical mill. When any of these processes are correctly done, lapping for a precise seal is unnecessary. Cost is usually about $20-30 per seat. With the YZF, the seats normally need little more than a light touch up, and very little material should be removed from the sealing face of the seat. The need replacement (assuming they haven't been damaged by excessive heat or some sort of impact) when so much material has been cut away to restore their proper angle and shape that it causes the valve to sit too far up into the head, so that it can't be adjusted to the right clearance.
4) OEM Yamaha springs are very reasonably priced, IMO, and are a good thing to replace just for the peace of mind.
5) Valve guides in engines using bucket tappets suffer no lateral pressures on the valve stem, as rocker arms would produce, and as such they shouldn't wear very much at all. This is the case with the YZF; the guides normally show virtually no wear even after long service, and don't ordinarily need to be replaced. Your machinist should check these in the process of refinishing seats, and advise you as to any that need attention.