I am working on a motorcycle engine and got into a discussion about how the valve clearances will change as the engine gets hot. It's on an inline 4 cylinder with 5 valve per hole. (small valves)

His claim was that the valve stem gets hot (expands) and reduces the clearance with the camshaft.

I disagreed since the head and other parts are also expanding (making it thicker).
Actually I feel the most important part is that the overhead camshafts spin in an aluminum "carrier" which grows much faster than the valves. The camshafts are probably about 1" of aluminum above the valve stem guides

The spec is for .003"-.005" clearance when "warm"
At the end of the day the thermal changes are probably pretty insignificant but I was interested in getting some other feedback.

Thermal changes in valve lash are almost certainly not insignificant.  They are critical on most engines I've seen.  note: valve lash is not the same as valve-to-piston clearance (just in case that's what you meant by "valve clearance")

Whether the clearance grows or shrinks will depend on the temperatures and materials of the various components involved.  The valves get very hot, but usually have a lower thermal expansion coefficient than the head (Al?)
valve, say nimonic 80a, 12.7 ?m/m-