The drill collars are made of 4145H modified chromium-molybdenum alloy steel according to API Spec 7 and aSY/T 5144 . All threads can be phosphated,copperised or cold worked. All threads are machined with CNC Lathes-Double-Shouldered and a special thread, the length as follow: 5FT, 10FT, 15FT, 20FT or according clients' request.