Thread type:Hex 25,Hex28,R32,,R38,R45,R51,H32,Hex35,T45,T51, Length:1000mm-6100mm extension rods,driftiing rods BETOPPER Drifting Drill Rod/Extension Drill Rod Extension rods are made by hollow drill steels, there are two shapes of extension rods, round type and hexagonal type,This hollow hole are usually named flushing hole, used for transmitting water or air during drilling.And the threads can ...