Because the consequence of too tight valve gaps can be serious, it is sometimes said that a ‘noisy tappet is a happy tappet’ (the noise – from the hammering parts – is a sign the tappet is not too tight).
But as we have seen, it is not quite that simple. When specifying the clearance gap for the engine Honda had to trade off two types of mechanical problem: damage from hammering parts (too much clearance) and damage from overheating (too little) .
In addition to minimising engine wear and tear, Honda also had to worry about performance. As we saw earlier, the clearance gap changes the valve timing (by delaying or advancing the point when the valve open or close) and this impacts the flow of fuel and air which in turn affects how engine runs.
The final factor is maintenance schedules (valve gaps change over time as the parts wear so need to be periodically adjusted).
It seems to be generally true that in engines that are well maintained there is relatively little wear on the tappet and rocker arm but the damaged caused by the clattering of valve against seat is unavoidable. Any resultant erosion of the valve seats causes the valves to sit higher in the cylinder head and this reduces the gap between tappet and valve end.
these consideration are summed up in this entry in a 1960s factory workshop for an older bike (the ca95):
Honda specify that gap for the 1978 C90 z2 should be set to a tiny 0.05 mm and recommend this is checked every 3000 miles.
Since Honda caused upheaval in the motorcycle market during the 60s and 70s based largely on their success creating low maintenance engines, you might wonder why – if it is true that the gaps tend to reduce over time, and given the serious consequences when gaps are too narrow – they did not allow a larger gap and then extend the maintenance interval? I suspect a clue to the answer is in the snippet about – a wider gap creates more engine noise, and this might have been the cause for returns to the dealer.