The Holte Percussion Underreamer is a two piece tool, a driver section & a bit section. The driver, or splined section fits into a down hole hammer.
The bit section has a pilot portion that is out of center to the rest of the bit and the drill string. As the hammer rotates, the pilot section tries to center itself. The friction of the nose of the bit on the wall of the well bore also helps pull the nose out from under the casing so it will drill a larger hole than the OD of the drive shoes on the casing.
The whole bit swings out and has a short heavy pin on the top side that fits into a pocket on the driver. A small portion of the air escapes between the wall of the driver pocket and the pin portion of the bit. This keeps the bit free from sticking in place so that a very minimum of reverse rotation will swing the bit back in alignment with the drill string for retraction.
The percussion underreamer along with a small compact Holte Casing Driver (which also serves as a diverter for water and air cuttings) will allow the casing to be placed easily in areas where it would have been a major struggle or next to impossible.
URE Underreamer is also available in a Drive Shoe Style for use without a Casing Driver. Both Eccentric Overburden Systems are available between 5" - 12".