Don:

A water cooled push rod operated overhead valve engine usually does have less running clearance on the exhaust than the cold adjustment, the exhaust valve is the hottest part, and expands the most. The intake is generally the same cold or hot.

Our model engines, having aluminum cylinder barrels, have more running clearance than the cold set clearance. Again, it is a matter of thermal expansion. The cylinder lengthens a lot more than the valves or push rods. This is why I always recommend their being set to 0.0005" clearance cold. On the Saito engines, at normal running temperature that leaves me with just about 0.002" running clearancce.

And while I would not object to 0.003mm cold clearance, I am sure, as BSC said, either you were told 0.003" or what was meant was 0.03mm. Just a hair tighter than the 0.04 stated minimum. If you use a feeler strip of 0.0015" you'll come out fine. That is 0.038mm, and you wont get it any closer.

Bill.