Junk sub is a short section of pipe that is located just above a bit or mill on a drillstring and is designed to catch large junk fragments in an outer bowl. Eddies in the circulating drilling mud cause the fragments to fall into the bowl. A junk sub is always run with mills and whenever drilling metal or cement. As many as three junk subs can be run on a string. Junk subs can have either long or short barrels. A junk sub is in contrast to a junk basket that uses reverse circulation and catches the fish on the inside of the tool.
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