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 be used for connecting couplings, shanks, couplings or bits. 
Usually for extension rods, there are R22, R25, R28, R32, R38, T38, T45, T51, ST58, T60 threads, and different lengths of rods are available from 600mm to 6400mm usually
(1)Steel grade: Male-Male Rod, MF Rod
(2) Thread: R22, R25, R28, R32, R38, T38, T45, T51, 
(3) Rod size: hex. 22mm, hex. 25mm, hex. 28mm, hex. 32mm, hex. 35mm, dia. 39mm, dia. 46mm. dia. 52mm
(3)Length: 0.4m-6m
(4)Package: in wooden case or in bundles. 
(5)Productivity: 20000MT / month