there is a model of bike that we sell at the shop...it has a bunch of burrs pretty far down inside the seat tube, so far that a standard hone does not reach. these burrs make installing and adjusting seat posts pretty unpleasant, plus the post gets really torn up.
so i made this extension for the hone to reach a smidge more than 12" into the seat tube.
it has an m6 grub for a set screw. i like to say grub. grub grub grub
the opposite end of the grub screw is stepped down to 3/8" to fit into the chuck of smaller drills.