By using small mirrors and gauges, a meaurement can be established which will determine the proper distance to within a thousands of an inch. It is necessary to make sure that the top caps of any pistom horn screw down to the same position everytime they are tightened. Any slack can cause the valves to be misaligned as well as possibly causing noise. A loose guide will cause a valve to be misaligned from side to side. Valve guides can be built up slightly to remove slop.
Valves have a natural tendency to twist in their respective casings.
This is directly related to the amount of pressure and air current flowing through the valve. Many times a valve will stick because of small imperfections along the slot which guides the valve guide. After fitting the valve guide, it is always a good idea to burnish the guide slot so that it is free of any restrictions.