An adaptor for drilling strings has a head, sleeve and mounting block assembly to secure a drill bit to a drilling string. By reverse rotation of the drilling string, the sleeve is movable between a first position, where pressurized air from the drilling string is directed to the drill bit, to a second position, where the air is directed to the drill hole above the adaptor to clear debris from the hole. Teeth at both ends of the drill assist the adaptor to clear the hole as the drill string is rotated and retracted from the hole.