I know the dimensions for flat head, 'double d'. I use standard ISO EN 5211. But what can i use for machining 'single' flat head?

Is just the same as ISO EN 5211 and only machine one side at the stem / shaft?

Being no expert on this, the general problem with valve/Forged Steel Valves stem top forms is that this is only 'sporadically standardized' and described.

Because of this you will find on the market, with differences from producer to producer, a larger number of stem forms, thicknesses, heights, orientations (parallell flow direction or 90 or 45 degrees turned), with or without key-slots etc. And even with this you have not started on the differences in describing all the different existing valve top flanges and borings for fastening gear or actuator.

If you do have access to the rules or regulations appliccable for the double D stem for your particular valve and this does not describe the single D, I doubt you will find a general valid answer for this another place.

Why single D?... or why not double D standard plus adaptor?

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