this surface 86 is preferably provided with a series of narrow v shaped grooves thereby to enhance the sealing effect the sealing ring 28 is preferably made of a tough polyurethane elastomer having a durometer hardness of approximately 85 it will be noted that the rod 44 has an elongated passage 90 extending longitudinally thereof from the extreme front end of the rod through a check valve 92 and thence communicating with a transverse passage 94 formed in the piston 54 this passage 94 communicates with the annular chamber 58 formed between the radially stepped portions of the piston 54 and cylinder 56 hence as hydraulic fluid under pressure is supplied through the passageways 90 and 94 the cylinder 56 is caused to move forwardly along the rod 44 thus causing the annular sealing ring 28 to be expanded radially outwardly in the manner described previously the annular seals 66 and 68 prevent leakage of hydraulic fluids between the rod 44 piston 54 and cylinder 56 during use of the packer it will be appreciated that substantial differential pressures are applied to the elastomeric sealing ring 28 ordinarily this tends to cause distortion and creep of the seal ring 28 in the axial direction in order to alleviate this problem a plurality of anti extrusion plungers 96 are provided these plungers being mounted in the rear portion of the main body section 40 just forwardly of the annular wedging surface 80 these anti extrusion plungers.