This is a small knob to accomodate a 3/8" bolt. The design on the picture uses a 2-1/4" long bolt and fit very nicely on my bike rack. The lenght of the bolt can be selected according to the project.

To install the bolt, drop the bolt through with the hexagonal head in the same orientation as the hexagonal hole. Using a nut, washer and possibly a spacer, carefully drive the bolt head down the hexagonal hole by tightening the nut. The bolt head is press fit.