Kelly Bar API Standard Oilfield Drilling Kelly Bar for Rotary Drilling Rig The drill collar is made of AISI4145H Cr-Mo steel and follows API Specifications and SY/T5144 standards. The threads can be processed with cold rolling, coppering or phosphating. Our company can machine double-shoulder threads and other specifications of special threads. The maximum outer diameter of the drill collar that can be produced by our company is 14 inches. The Company possesses a complete set