You should be talking of clearances, NOT tolerances and the clearances when the engine us Running. is what matters. 4 Valve heads are near impossible to cool effectively without liquid cooling as the 2 exhaust valves in each head are very close together. but are NOT needed. Good U/L engines shouldn't need to be complex. Liquid cooled engines run cooler than optimum for efficiency and Play up more in hot Ground temp environments. Ie Overheat on the ground if time is Prolonged. The also have the drag of radiators and vulnerability of Leaks and extra complexity, COST and weight and need to check Hoses, radiator Mounts water Pump seals etc and you don't want head gaskets and joins, IF they can be avoided. Nev