Abstract: An improved plug mechanism for use with a wellhead tool for removing and replacing valves on an operating wellhead under pressure including an elongated housing with a fluid coupling on one end for connecting to the outlet end of a wellhead valve and a plug mechanism extending from the housing through the valve into the wellhead where it seals fluid flow to the valve. The plug mechanism includes an expandable sealing element and an improved latch mechanism including a plurality of ball bearings which are extendable and retractable from the plug mechanism to secure it solidly within the bore of the wellhead upstream of the valve to be removed. The housing is equipped with an operating mechanism to expand and retract the sealing member and to engage and release the ball bearings. The plug mechanism further includes an improved self-alignment feature to ease attachment of the operating mechanism to the plug mechanism once the plug mechanism has been secured within the bore of the wellhead.