Junk subs are constructed with rib

Junk subs are constructed with rib guides which are welded to the main body. These guides assist in preventing the cup from becoming crushed and guide the tool. Junk subs are available in various lengths.
Guidelines
The following guidelines shall be used when running junk subs:
 The mud stream agitates the junk and causes it to rise as drilling or milling proceeds.
 The annular clearance is small at the bottom of the sub (resulting in high annular flow rates which lifts the junk off bottom) and larger above the sub (resulting in smaller annular flow causing the junk to fall into the cup of the junk sub). Vary circulation rates from maximum to zero while reciprocating string.
 The junk is retrieved when the bit or mill is retrieved.
 In packer milling operations, junk subs can be run in tandem to handle the large volume of junk produced by the milling operation.
 Junk subs may be run above a casing scraper to implement quicker, cleaner scraping jobs.
 In completion fluids, a viscous pill may be required to lift the junk into the cups of the junk sub.
 Confirm junk subs have been inspected and certified before use. Beware of ‘rig-made’ junk subs.
 Care should be taken when fishing for heavy junk. The weakest part of the junk sub is the top sub and twist-offs are possible.