Extension rods are made by hollow drill steels. 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. 
 
Body Type:
There are two shapes of extension rods, round type and hexagonal type. 
Hexagon rods are more rigid, heavier and transfer energy more efficiently, while increasing flushing.
Round rods are generally lighter than hexagonal rods and are normally used in extension drilling applications.
 
Thread Type:
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) Type: Male-Male Rod, Male-Female 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.