Medium Rudder Button Zinc 3-3/4"x 3/4"( RB-M ) Deep slotted stainless steel machine screw for maximum torque. Unit comes with both halves and screw/bolt making a full complete clam. Medium mounts with 1/4"-20X2'' machine screw. Rudder Plates are designed in order to be streamlined, minimizing drag and turbulence.