Originally Posted by San Rensho
Second Timcuppery, but mine is even simpler, just two very big washers that fit over the ends of the cups and a long bolt and nut. Grease well, makes it easier to install and more importantly, remove.
The problem with washers is that they place the force on the outside of the cups. This is fine with most cheaper steel headset cups, but not with lightweight headset cups where the aluminum cups have steel races. Ideally, when pressing in a headset, you want the force distributed in line with the part of the cups that fits into the frame. Bushings that fit closely do this. Washers don't.