referring now to the drawings , and particularly to fig1 - 4 , a preferred embodiment of the apparatus of the present invention is shown and generally designated by the reference numeral 10 . in fig1 , a schematic diagram of a typical well 100 is shown . the well 100 includes sections of casing 102 installed into concentric wellbores 104 , such as a conductor casing 106 , a surface casing 108 and an intermediate casing 110 . it should be appreciated the term casing used herein also includes liners or any other form of a tubular member . the casing 102 extend to varying depths according to the engineering of the well 100 and particularly to the different formations through which the wellbores 104 extend . the well 100 further includes a deviated or horizontal wellbore 109 into which a production casing string 112 is being run in . with reference to fig2 , an apparatus 10 , constructed in accordance with the present invention , is shown engaged with the casing string 112 and positioned within a previously installed section of casing , such as the intermediate casing 110 . the casing string 112 is generically illustrated for exemplary purposes only . the apparatus 10 is a piston assembly 10 including a piston body 12 having a forward end 14 and a rearward end 16 . the piston body 12 further defines a radial shoulder surface 18 , which is of a diameter to correspond to the i . d . of the intermediate casing 110 such that a sealing contact is made between the radial shoulder surface and the intermediate casing . the radial shoulder surface 18 includes a wear surface 20 and a sealing surface 22 . to aid in centralizing the piston assembly 10 , a centralizing member 114 can be engaged with the casing string 112 , and depending upon where the piston assembly 10 is engaged along the casing string 112 , a centralizing member can be used forward and rearward of the piston assembly . the centralizing member 114 is a mechanical device to position casing concentrically in a wellbore . usable centralizing members 114 are well known in the art and do not need to be described herein . preferably , the piston assembly 10 is engaged with the casing string 112 between the liner hanger , not shown but well known in the art , from which production casing depends ( not shown ) and the second to the last joint of pipe , in the casing string , required to land the liner hanger in its engineered well position within the intermediate casing 110 . now with reference to fig3 and 4 , a partial cross - section of the piston assembly 10 engaged with the casing string 112 and positioned within the intermediate casing 110 is shown . the piston assembly 10 further includes a mandrill 24 and an adaptor sub 26 . the adaptor sub 26 and the mandrill 24 allow connection of the piston assembly 10 with casing string 112 . the mandrill 24 is slidably received through the rearward end 16 of the piston body 12 by a longitudinal bore 28 . the mandrill 24 is positionable in a first forward position as shown in fig3 and in a second rearward position as shown in fig4 . the mandrill 24 is positioned in the first forward position during running in of the casing string 112 and then positioned in the second rearward position upon completion of running in the casing string . the mandrill 24 when positioned in the first forward position , as shown in fig3 , the first end 30 thereof is engaged with the adaptor sub 26 as shown . preferably , the first end 30 of the mandrill 24 is threadably engaged with the adaptor sub 26 . the piston body 12 further defines at least one blind port hole 32 formed therethrough from an exterior surface 33 forward of the radial shoulder surface 18 to the longitudinal bore 28 , and at least one annulus port hole 34 formed therethrough from an exterior surface 35 rearward of the radial shoulder surface to the longitudinal bore . preferably , four blind port holes 32 and four annulus port holes 34 are formed through the piston body 12 radially therearound and offset at 90 degrees . the blind port holes 32 and the annulus port holes 34 a restricted from fluidic communication when the mandrill 24 is in the first forward position , as shown in fig3 , and are in fluidic communication when the mandrill is in the second rearward position , as shown in fig4 . the piston assembly 10 further includes at least one projection 36 extending into the longitudinal bore 28 approximate the rearward end 16 of the piston body 12 . preferably , there are two opposed projections 36 . most preferably , there are four projections 36 spaced radially around the rearward end 16 . the projections 36 can be spaced radially around the rearward 16 at an offset of 90 degrees . a cooperating slot 38 formed longitudinally along the exterior surface 39 of the mandrill 24 receives each projection 36 . each slot 38 is formed along the exterior surface 39 of the mandrill 24 , such that when the mandrill is in the first forward position , each projection 36 is free from engagement with the cooperating slot 38 and the mandrill is free to rotate about its axis within the longitudinal bore 28 . and such that when the mandrill 24 is in the second rearward position , each projection 36 is engaged with the cooperating slot 38 and the mandrill is rotationally locked with the piston body 12 . a piston body 12 further defines at least one longitudinal passage 40 or channel formed through the radial shoulder surface 18 from the annulus side or rearward side of the radial shoulder surface to the blind or forward side of the radial shoulder surface . a check valve 42 is fitted within each passage 40 and allows fluid to flow from the blind side of the radial shoulder surface to the annulus side of the radial shoulder surface when pressure within the intermediate casing on the annulus side is below a predetermined value . additionally a plurality of o - ring seals 44 can be positioned along the longitudinal bore 28 to providing sealing between the mandrill 24 and the interior surface of the longitudinal bore . in use , it can now be understood , the piston assembly 10 is engaged or otherwise installed inline in the casing string 112 between the casing hanger from which the casing to be installed depends and the second to the last joint of casing string pipe required to land the liner hanger in its engineered well position . the casing string 112 with the engaged piston assembly 10 is inserted into the intermediate casing 110 using surface equipment well known in the art to a predetermined point . the casing string 112 can be inserted into the intermediate casing to the predetermined point through such methods as gravity feeding or floating , both well known in the art . while these two methods are mentioned , it should be appreciated that other methods can be used to insert the casing string 112 . during this process , fluid is free to flow from the blind side of the piston body 12 to the annulus side of the piston body through the passages 40 . once the casing string 112 with the engaged piston assembly 10 is inserted to the predetermined point , the intermediate casing 110 above the radial shoulder surface 18 is pressurized with a fluid , such a drilling fluid . the intermediate casing 110 can be pressurized with drilling fluid through known techniques and equipment . one such technique that is possible is through the use of drilling fluid pump connected to a source of drilling fluid and to the wellhead through a kill line and other conventional equipment . the pressurization of the intermediate casing 110 creates a pressure differential across the piston assembly 10 and forces the piston assembly , which is engaged with the casing string 112 , to translate through the intermediate casing and into the wellbore 110 , thereby pressing production casing attached to the casing string 112 into the wellbore . once the casing string 112 has reached a position within the intermediate casing 110 that facilitates landing of the production casing to the intermediate casing , the intermediate casing is depressurized . the pressure cross the piston assembly is then equalized by establishing the blind port holes 32 in fluidic communication with the annulus port holes 34 by moving the mandrill 24 from the first forward position to the second rearward position . once pressure is equalized across the piston assembly 10 , the piston assembly is backed off from the installed production casing and withdrawn from the well 100 using known methods . a number of embodiments of the present invention have been described . nevertheless , it will be understood that various modifications may be made without departing from the spirit and scope of the invention . accordingly , other embodiments are within the scope of the following claims .