casing hanger 39 when the engagement element 37 is engaged with the casing hanger 39 although not shown a string of casing is attached to the lower end of casing hanger 39 the inner surface of the engaging element 37 is initially in contact with the threads 19 on the inner surface of stem 13 a piston 41 surrounds the stem 13 and substantial portions of the body 25 referring to fig3 a piston chamber 42 is formed between upper body portion 27 outer sleeve 30 and piston 41 piston 41 is initially in a and upper or u201c cocked u201d position relative to stem 13 meaning that the area of piston chamber 42 is at its smallest possible value allowing for piston 41 to be driven downward a piston locking ring 43 extends around the outer peripheries of the inner surface of the piston 41 locking ring 43 works in conjunction with the latch device not shown contained within outer sleeve slot 32 to restrict movement of the piston during certain running tool functions a casing hanger packoff seal 45 is carried by the piston 41 and is positioned along the lower end portion of piston 41 packoff seal 45 will act to seal the casing hanger 39 to the wellbore not shown when properly set while piston 41 is in the upper or u201c cocked u201d.