I've set a bunch O valves and a 10mm offset box, hold the stem with a needle nose works fine. I think I measured the square on the end of the stem at 3mm once. If you really need a wrench take a cheap stamped ignition wrench or other piece of say about 1/16 strap drill a small hole then drive the square tang from a small file into the hole with a hammer till its a good fit on the square. Did that once, lost it and just went back to a needle nose. Normally if the nut turns freely on the stem thread I just set the gap a titch wide and as the nut tightens, it closes the gap a little and it's set.