When converting a frame with vertical dropouts for use with a singlespeed, fixed-gear or internal hub, there's often a derailleur hanger left out in the cold. Our stainless steel plug fits in there nice and snug and protects the hanger threads from dirt and grime. We've put the 4mm broach on the bottom of the bolt (not the head) so that it threads in from the inside for a clean, finished look on your drive-side. Nice.