Description:Cross-over joint acts for conversion and connection of drill stem component in petroleum ,natural gas and geology drilling work.It is used mainly for connecting of upper drill tools and lower drill tools during drilling operations. It is divided into drill string cross-over joint and tubing cross-over joint.There are A type (same diameter),B type (different diameter) and C type. Bit subs are used to connect the drill bit to the next piece of equipment in the BHA. Because drill bits are "pin up" and all other drilling components are typically "pin down", the bit sub functions as a crossover for BHA attachment. Kelly saver subs Placed between the kelly and the drill string, this sub is used to extend the life of the kelly by limiting the number of times it's pin connection is made up to the drill string. This saver sub provides a connection between the pin end of the kelly to the box end of the drill string such that the repetitious connecting of drill pipe is absorbed by this sacrificial saver sub.