A system for adapting top head drilling rigs for reverse circulation drilling includes a generally tubular adapter having an upper end which is threadedly received in the drive head of a top head drill rig. A collar is secured to the tubular adapter by a pair of rotary bearings and includes an exhaust...http://www.google.es/patents/US4271916?utm_source=gb-gplus-sharePatente US4271916 - System for adapting top head drilling rigs for reverse circulation drilling
System for adapting top head drilling rigs for reverse circulation drilling
