The Holte Manufacturing Co. URG three-arm underreamer was designed for large diameter drilling to enable underreaming on all sides rather than only one side. The pilot bit is large like a conventional down hole bit, leaving less area for the arms to ream. Additionally, the concentric URG drills are said to drill well in boulders because the reaming surface is extended on all sides, attacking edges with stability. Holte makes reverse circulation URG underreamers as well as standard. Holte Manufacturing Co., Card No. 305