I believe that under more extreme situations, possibly max load or maybe even max closed loop load that it's quite possible for valve clearances to be very close to zero.
My theory is that under operation the average temp over the length of the valve could easily be a couple hundred Celsius above the average temp of the head. Calculating this out on a given head this alone eats up a good portion of valve clearance in CTE.
In some searching I wasn't able to find anything that specifically discussed this. I did however find this study that I believe supports my claim. http://www.ww.journalamme.org/papers_vol23_2/1122....
Unfortunately it is done on diesel engines. I know a lot less about Diesel but it's my understanding that generally combustion temps and EGTs are cooler correct?
In searching older threads here I saw people suggest that the head of the exhaust valve on a performance gas engine could get close to 800C which doesn't seem too surprising to me since it's not abnormal to see 900C EGTs many inches away from the exhaust valve. In this case even if the average temp was much lower it wouldn't be hard for thermal expansion to take up all of that valve clearance.
It's my theory that when calculating for valve travel, required valve spring travel or anything like this that you should assume that in the right circumstances the valve clearance could be very close to zero. Does that sound right?