Why Are There Clearances To Check?
The clearance is between the cam (egg shaped lobe which presses on the valve spring to open the valve), and the bucket which sits underneath it. Old engines used to use tappet adjusters which could be adjusted with a screwdriver, spanner and a feeler gauge. Modern bikes use bucket and shim. The bucket is under the cam, and the shim sits underneath the bucket. By changing the thickness of the shim, you maintain the optimum gap, and make sure the cam opens and closes the valve at the right time.
You will notice that motorcycle service manuals state that valve clearances must be checked when the engine is cold (after the machine ideally is left to cool overnight). This is because the valve clearances are smaller when the engine is hot. The exhaust valve gets hotter than the inlet valve, so a greater tolerance is allowed due to thermal expansion on the exhaust valve. When the engine is hot the tolerances should be virtually zero. The difference, then, between an up to temperature engine and a cold engine is approximately 0.15mm on the inlet valve and 0.3mm on the exhaust (modern Japanese bike). The exhaust valve gets hotter than the inlet valve and therefore expands more, so needs a greater cold clearance.What happens if the valves are too tight? Well, at cold it’s not a problem, but as the engine warms up and the tolerances are taken up, problems arise. If you hear about burnt valves, this is what is happening; the exhaust valve which sits directly in the stream of hot exhaust gases at extreme temperatures relies on the direct contact with the valve seat to transfer heat away and cool itself. Take away that cooling and the valve gets too hot and eventually starts to break down.
Burnt exhaust valves are an extreme case, and for this to happen, your valves will be in a bad state of alignment, requiring a serious tune up. If you are mechanically sympathetic, you shouldn’t let your bike get this far. The likely issue with tight valves is one of valves opening too early, and closing too early and in extreme cases not seating properly. The not seating properly gives you leakage when you want a tight seal, and a leaky engine ultimately means reduced power (the reason we use a compression test to check not only piston rings, but cylinder head condition).
What about loose valves? Well, the tolerances can get bigger over time, usually due to valve train (cam) wear, but far more likely in the early life of an engine is for the valve clearances to tighten up as the valves ‘bed in’ to their seats. Valve clearances which are over the recommended tolerance may manifest in increased noise, (no contact followed by contact between cam and follower) and in extreme cases, the valves open later and close earlier and don’t open as much, reducing the volumetric efficiency of the engine. Somewhere in between too tight valves and too loose valves lies the designed valve tolerance, optimum operation, and this is what your checking and adjustment is aiming at.