to compensate for wear of the guideways or of skid surfaces 43 and to ensure that its top and side sealing element are properly positioned for sealing another form of the present invention is shown in fig6 and 7 as ram 50 ram 50 has an exterior shape to fit closely in the guideway in which it is to be installed slot 52 across the rear of ram 50 is provided for the attachment of the connecting rod not shown recess 54 on the front face of ram 50 is provided to receive tubular members therein mud slot 56 on the lower surface of ram 50 includes rear recess 58 and forward slot 60 which is narrower than recess 58 as shown in fig7 skid 62 is positioned in recess 58 and extends into slot 60 skid 62 includes body 64 with y shaped slot 66 extending therethrough as shown with the double slots 68 opening on the rear skid 62 is secured to ram 50 by screws 70 which are positioned in recesses 72 the heads of screws 70 are positioned wholly within recesses 72 so that they never engage the guideway in which ram 50 is positioned to adjust for wear one or more shims 74 is positioned in slot 56 skid 62 provides a pair of side skid surfaces 76 and triangular.