Its main function is to ream the hole to the correct constant specific diameter which ensures adequate clearance for the core barrel and sufficient clearance for the new bit which replaces an old one. It also acts as a stabilizer for drill bit.

Reaming shells (with double pads or three pads) are suitable to use in all formation conditions ranging from broken and abrasive to consolidated and non-abrasive. As this kind of reaming shell is physically longer than the standard type, it is necessary to use an appropriate inner tube extension in order to compensate for the difference in overall reaming shell length.