Thread type:Hex 25,Hex28,R32,,R38,R45,R51,H32,Hex35,T45, ... , round type and hexagonal type,This hollow hole are usually named flushing hole, used for transmitting water or air during drilling.And the ... -T513660T51-T511220T51-T511525T51-T511830T51-T512435T51-T513050T51-T513660 T45 (1 3/4") Drill Rods for Drilling machine