I've bought a flat-pack bed second hand, and it has one part missing. There are two boards I need to join, and according to the instructions I should hammer a short (about 4 cm) hollow metal cylinder into the hole in one, and then push it into the other, and then put a bolt through the cylinder (and fix it in place with a barrel nut). The cylinder is the missing part and I'd like to know what it's called so I can find a replacement. It's smooth both inside and out.
Here's a sketch of the situation in section, which is hopefully clearer: (I've omitted the barrel nut)