For extension rod, there are two shapes, one is round type, another one is hexagonal type. All extension rod are threaded and made by hollow drill steels.
This hollow hole are usually named flushing hole, used for transmitting water or air during drilling. Two ends threads can be used for connecting couplings, shanks, couplings or bits. Typically two ends of extension rods are male, while sometime one end is male and another one is female, then we call it M/F rod. We need couplings to connect extension rods, while for M/F rods, because one end is female, so couplings are not necessary.
1) Thread type: R25 R28 R32 R/T38 T45 T51 T60, etc.
2) Length: 600mm-7000mm
3) Rod type: hex or round(plain)