This locking seatpost bolt will prevents removal of your seat post from your frame. Designed to keep your expensive seatpost where it belongs. An ordinary wrench can not grab onto the specially shaped nut and bolt heads. Only the included wrench can engage the heads. The wrench is designed to work with both this binder and our Locking Track Nuts. 20mm and 28mm are the usable clamping distance between the outside of these two sided. Generally with bike clamps, you are squeezing two sides together. Please study bolt drawings in this listing to determine if this bolt will work on your bicycle.