Originally Posted by OLDYELLR
How would that be? Not actually having had one apart, I'd assume you just push the stem clamp thingie down until it stops and tighten the allen screws, making sure the bars are straight. How precise is that? When I adjust my hubs after repacking, I turn the cone a fraction of a turn at a time to get the clearance exactly right. Same with the adjustable cup of my bottom bracket. It's a micrometer-like adjustment. A threadless headset seems agricultural in comparison.
Now, roller bearings are something that make sense. They started using them in some motorcycle steering heads 40 years ago.
The load on the bearing is set by the torque on the top cap. The top cap forces the stem and spacers down on a wedge around the steer (in most cases) so the load is adjustable in fractions of a screw turn at the top cap. Pretty fine adjustment and certainly finer than the old jam nut system of threaded headsets.