SUBSTITUTES
A crossover substitute joint, often called a sub, is an adaptor to connect two (2) components when their threads do not match. Sometimes it is necessary to combine two (2) subs in order to make the connection. A special sub, called the kelly sub, is used at the pin end of the kelly to save the kelly from undue wear. As few substitute joints as possible should be used in the drill string because most pipe failures occur at the joints. Joints should be made up tightly.
