I hate to chime in on threads like these because they always turn into "well, I was taught...", but against my better judgement, I just can't help myself sometimes. LOL
From 1973 until 1981 I was a Kawasaki and Suzuki mechanic. From 1983 until 1989 I worked solely for Suzuki. In those years I went to all the OEM seminars these manufacturers had from the Z-1 (1973) to the GSXR 1100. In that time period, the three "main" types of OHC valve opening systems that we have currently (rocker arm, shim over bucket, and shim under bucket) were all in use in various engine configurations. I quoted "main" because I have no experience with pneumatic and desmodromic systems. For the last thirty years, I have mostly fiddled with my own herd, and friends bikes, and for a couple of years, a roadrace team.
This is what WE were taught by the companies that actually build these things...
Rocker arm operated valves...
1)Need clearance attention the most.
2)Have specific settings (not a range).
3)Generally gain clearance over time.
4)Are checked / adjusted by inserting the correct feeler gauge for the specific clearance. If the gauge goes in with no effort, or will not go in at all, the valve should be adjusted to spec. To accomplish this, the adjuster screw on the rocker arm should press the feeler gauge against the valve stem in such a way that AFTER the locknut is tightened, there is enough resistance in the feeler gauge to feel snugness moving it back and forth.
It is generally accepted that the wear between the cam lobe and rocker interface and the rocker arm adjustment screw and valve stem, are greater than the wear of the valve face and seat, thus the clearance increasing.
ALL SHIMMED valve clearances are a "setting" range, NOT an "operating" range. I think some people need to let that sink in. What that means is the engine manufacturer is saying that you can set a shimmed valve at the MINIMUM of the clearance range, (or MAXIMUM for that matter) given in their specs., and you are in no danger of engine damage from a valve getting too tight or too loose for the duration of the valve adjustment interval.
In shim OVER bucket systems the clearance range is narrower and tighter, and the clearance check intervals are more frequent than shim UNDER bucket systems. The reason for that is two fold. In shim over bucket valve trains, the clearances tend to increase between intervals, about 80 percent of the time, and with too much clearance the chance of spitting a shim is greatly increased. Also, may engines with shim over bucket valve trains did not have rev limiters which was another cause of spitting shims. It is generally accepted that the wear between the cam lobe and the shim is slightly greater than the valve face and seat which lead to clearances increasing. However, engines that were revved to the point of occasional valve float, would see their valve clearance decrease over time because of the hammering the valve face took, if they didn't spit a shim first. LOL
Which brings us to the most dependable and durable of the OHC systems, the shim UNDER bucket. Nowadays, with captured shims, rev limiters, and water cooling commonplace, valve clearances can be far greater than ever. This is the main reason for extended valve check intervals.
More likely than not, when a shim measures .001" less than the minimum setting at valve check time, that valve was probably at the minimum clearance range out of the factory. From my experience, I would bet that if left alone until the next clearance check, it would still have the same reading. None of these engines are in any danger of valve damage unless their clearance reaches zero, and I've yet to see that happen in a shim under bucket system, some in excess of 150,000 miles with no adjustment, ever! I'm in no way advocating a no maintenance approach to your beloved steed, but I think there is WAY too much hysteria when it comes to a valve that is .001" tighter than the original minimum.
Furthermore, a valve adjustment interval is arbitrary at best. No one knows how someone is going to operate their motorcycle. What a valve adjustment interval is based on, is the amount of estimated revolutions an engine is going to turn before the manufacturer says the valves need checking. The same manufacturer that wants you to change your plugs at 12,000 miles, your hoses every four years and a whole host of other things that most owners scoff at and think is way too soon. Let's create an extreme case scenario. Rider#1 likes to do all riding in first gear only. Rider #2 loves to get into sixth gear as soon as possible and do all riding in sixth gear. They now both have 24,000 miles on their bikes but rider#1's bike has turned DOUBLE the engine revolutions in the same mileage. Go figure.
So what's the paranoia about valves? If, and that's a big if, they are getting too tight, you will have a ton of warning with the bike becoming hard to start and running crappy, before any burnt valves are in the picture.
