A 20mm long hexagonal cylinder, 6mm across the flats. My MegaPro ratcheting 13-in-1 screwdriver uses double-ended two inch bits. I wanted to use it with some single-ended one inch bits, so I made this drop-in spacer. It is slightly undersized so it won't get stuck in there. Update: It turns out, even while being slightly undersized, there is a chance that if you put enough pressure on the spacer, it will deform and be difficult to remove. It also turns out that one of my earlier designs works just as well for this use case: http://www.thingiverse.com/thing:350118 Just customize it to "adapt" 1/4" to 1/4" bits.