Depending on where the valves or stems are produced, type of valve, and to what norm and market. For European produced valves to EN standards you will find other standards commonly used also for stems. Power screw threads are normally used because the purpose of the stem is to raise or lower the control element or obturator inside the valve. The surface angle and thread height of ACME type threads generates less contact stress and wear than UN. Also, with UN and other bolting type thread series, it will take many more turns to move the valve to open or close. Naturally there are exceptions and special thread series where it makes sense to use, but in general, ACME power threads are used on multi-turn linear Automatic Control valves. From memory, ACME was the first power thread standard which many countries simply adopted after WWII.

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