These kits let you drill pipe and tubing with a minimum 3" diameter and some convex surfaces. Kits quickly attach to your Hougen Magnetic Drill and positive non-slip positioning assures centered hole location. Minimum 3" diameter schedule 160, maximum 12" diameter schedule 160. Chain mount for models HMD904, HMD505, HMD905, HMD914, HMD917, HMD918, HMD920