Entry Sub for Drilling Bumper Sub Kelly API Drill String Adapter The drill string adapter is made of AISI4145H Cr-Mo steel and follows API Specifications. The Company can produce same-diameter, reducing and left lateral drill string adapters and can perform cold rolling, coppering or phosphating of threads. The Company possesses a complete set of petroleum drilling tool production lines and mainly produces large deep-hole boring machines, ordinary lathes, CNC lathes, mill,