Well, apart from the fact the once done, the headset will stay on the correct setting (you may need to adjust it once after the installation). The star nut cannot slip, it's a fact.
But if you cannot drive it properly, or you have the wrong size; as long as you fork crown is "free" at the bottom, just put a pice of M5 threaded bar with two nuts, tighten the h/set, close the stem and remove the threaded bar.
Well, apart from the fact the once done, the headset will stay on the correct setting (you may need to adjust it once after the installation). The star nut cannot slip, it's a fact.
But if you cannot drive it properly, or you have the wrong size; as long as you fork crown is "free" at the bottom, just put a pice of M5 threaded bar with two nuts, tighten the h/set, close the stem and remove the threaded bar.