19-01-2020, 12:16 AM
Logically the gap should be the smallest which guarantees full closure of the valve when thermal expansion is take into account i.e. the hotter the valve runs the bigger the gap needs to be (hence wider gaps for exhaust valves than for inlets, which are cooled by the incoming charge rather than heated by the outgoing exhaust). I'd venture that racers are more likely to push it to the limit and tourers will tend to increase the gap to be on the safe side. On the other hand the more fire it breathes the bigger the gap wants to be...