valve clearance always the same?

[ThomasSwiss]

I'm interested in, does the valve clearance always remain the same, whether standard, tuning or blown engine?

[Chris KC]

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...

[Bob Culver]

I note that everyone now seems to adopt a greater clearance than both Austins originally and the revered special book writer Williams recommended. Presumably modern exh valves are of modern material which expands more than XB and earlier steels.  (Unless valves are of two piece welded construction)
Valve clearances for "modern" cars are very different because these have opening and closing ramps to reduce noise, impact and sensitivity to wear.