15mm Diameter. Round shaft. Phenolic with set screw. This is a direct replacement of the knobs included with most kits. Note: These and the 1900 style knobs are the only knobs we offer at this time to fit the following kits: OD2, 5 Knob Comp, Ping Pong Delay, Envelope Filter, Phase Royal, Parametric EQ, and Shredder due to limited knob space. Please be aware of this before ordering custom replacements.