and with bearing surface 62 seating on face 52 to place the lock segments 12 into a locked position in the locked position locking head 36 extends radially inward and beyond the inside diameter of body 20 and into counterbore 64 on sleeve 22 counterbore 64 includes shoulder 66 that as sleeve 22 is moved downward relative to body 12 engages concave surface 60 and is prevented from further downward relative movement referring still to fig1 and fig5 shoulder 66 of sleeve 22 and concave surface 60 of lock segment 12 preferably extend at an angle less than 45 degrees from horizontal such that the majority of the force applied by sleeve 22 onto lock segments 12 is projected downward through the lock segments 12 the downward projected force carries through bearing surface 62 of lock segment 12 onto face 52 of body 20 any horizontally directed loads are directed from convex surface 58 onto concave face 50 once lock segments 12 are engaged they cannot be moved radially thus providing a positive locking engagement between body 20 and sleeve 22 that will not be disengaged by increasing loads from sleeve 22 the load created by the downward movement of sleeve 22 is carried in shear across each locking segment 12 which individually and collectively are capable of carrying significant loads referring now to fig6 the locking mechanism 10 is unlocked by hydrostatic pressure in the interior 68 of tool 18 cavity 24 is.