That's actually an old stem nut that was on the bike when I first got it. It is strangely "double thick" with that bushing-looking bit, but it is all one piece. When I changed the tires and tubes, I managed to misplace the new nut and re-used the old one. If I put the new knurled stem nut on like you see in the earlier pic of the tube, it would give me even more clearance in that area and I'd have over 1/2" protrusion. I don't know why the tubes from earlier in this thread apparently come with (2) nuts except so that they could be used to 'lock' one another, but that takes up too much thread space.....