described above the under reamer 14 is used to mill out the well casing at the specified depth the under reamer upper body 14 e consists of a mandrel having appropriate threads eg a 2 u215c u2033 api pin thread on the top this api thread threads into the bottom of the ljs 12 the mandrel threads into an under reamer lower body 14 d as known to those skilled in the art the under reamer will preferably include a set of backwards facing wash jets to divert some of the drilling fluid to the outside of the under reamer this fluid is used to wash milled chips into the sump of the well the piston 14 a applies pressure to deploy the milling arms under hydraulic fluid pressure such that a differential is created between the piston and the under reamer lower body the piston sits on compression spring 14 b that is used to return the piston to its retracted state after milling is completed the milling arms 14 c are knife arms with carbide inserts on both the top and bottom sides of the milling arms the milling arms are pinned to the under reamer lower body and can pivot about this pin the top of the ljs has appropriate connector threads such as a 2 75 stub acme box thread that threads into the bottom of the crossover sub 12 at the top of the tool string the bottom of the ljs has a 2 u215c u2033 api thread that threads into the top of the under reamer tool 14 the bullnose 16 has a 2 u215c u2033.