There are no less than 5 threads going on right now about adjusting valves. It is my assumption a short class on valves is in order and may clear up misconceptions and misinformation. I don't claim to be a know-it-all or an engineer. I do have many years of practical experience and to this day remember the a-ha times that I learned how valves worked. Any one with better knowledge, please chime in. I hope this helps.
The shims (in various thickness) are a design feature to allow adjustment of valve lash. This is also called the measured clearance between the cam lobe and valve stem. There are lots of ways to accomplish the same task. The cam can be in the bottom of the engine and push on the valves with a pushrod/rocker arm (HD, Triumph, Chev 350 V8). The cam can be in the head and push on the valve with a rocker arm (Honda 50/70/90/350/450/750, KLX110) and the cam can push directly on the cam (YZF, Most modern 4 stroke street bikes)
Rocker arms have a threaded adjuster(tappet) with a lock nut. Very easy to adjust, but has problems at high RPMS. (Mass and momentum of components does not change directions very well) Higher performance means overhead cams, pushing directly on the valves. If materials did not wear or change size when heated/cooled and manufacturing tolerances were +- zero, then no adjustment capability would be needed. Because these things do happen a method to adjust is needed and thus we have shims in various sizes.
Due to manufacturing tolerances, no two engines assemble the same. You know, a thousandth here and couple thousandth there and you are talking serious gaps.
Another factor at work is that in the first hours of operation, engine parts seem to settle in to their comfortable places. Kind of like the difference between new boots and ones that are broken in. In the case of valves this usually means that the valves settle farther into the valve seat. (The valve seat itself may even settle into the head, a ten thousandth or so) All this combines to let the valve get closer to the cam lobe, reducing or shortning the lash/gap. If the valve lash were zero the cam would hold the valve open a small amount no matter how the cam is positioned. In most cases on machinery, tighter is better, except on valves. You will get better performance, compression and gas mileage if the valve lash is adjusted to the loose limit.The result of all this theory is: even with all new components, you have to measure and adjust valve lash. You should check and adjust as needed after the first few hours of operation. Street bikes used to be at 600 miles. Yamaha says 1.5 hours on the YZF. In my mind some where around 5-10 hours is good. Then on a semi-annual/annual basis depending on use. A factor that really helps the maintenance negligent, is that modern materials stretch less and machine work is more precise from piece to piece so that valves do not need as much attention.
As far as the procedure, Read the manual. Measuring is easy. Rotate the cam until the sharp pointy end of the cam lobe is pointing away from the valve you want to measure. If you get an actual measurement (feeler gauge drags but does not need to be forced) then you can do some math and select the correct shim the first time. After putting every thing back together, rotate the engine over several rpms, to squeeze out any oil taking up space between the valve/shim/cam. Then measure.
As with any maintenance, clean the bike/engine first, lay your parts out on a clean cloth, be meticulous, and follow the directions. That's all. I have to get back to work.
Don
It gets much easier on the second measurement, it's just that easy.
