arc to extend and retract arm 88 with arm 88 in its retracted position surface 91 is adjacent camming surface 85a in this configuration the overall diameter of the bit assembly is less than the inner diameter of the bore casing permitting the bit assembly to be withdrawn from the bore as arm 88 is rotated clockwise about pilot bit 82 by clockwise rotation of bit driver 52 angled surfaces 85a engage surface 92 and urge arm 88 outwardly the rotation and extension of arm 88 continues until surface 92a is abuts surface 85b and surface 92b abuts surface 85a locking arm 88 in its extended position to unlock and retract arm 88 bit driver 52 is rotated in the opposite direction in its fully retracted position the overall diameter of the underreamer assembly is less than the inside diameter of the casing permitting withdrawal of the entire underreamer bit assembly through the casing if necessary this feature represents a significant advance over known underreamers which cannot be retracted and withdrawn through the casing if necessary in operation compressed air is delivered into annular chamber 59 through port 37 radial ports 60 annulus 62 and axial ports 64 in fig5 hammer 53 is shown during its downward stroke lip 66 is engaged.