Abstract: An improved tool for installing captive type spark plug gaskets upon a spark plug body characterized in the provision of a staking member which contacts the inwardly directed locking flange of a gasket in such manner as to form undulations in the plane thereof to provide the equivalent of metal thickness which is appreciably thicker than the original thickness of metal stock from which the gasket is formed. During a staking operation, the inwardly directed flange is further inwardly folded to reduce the effective diameter of the free edge portion thereof, so as to be positioned to lie upon the unthreaded undercut area adjacent the lower transverse surface of the plug body. Because of the effective increased thickness of the free edge of the inwardly folded flange, cross threading with respect to the thread of the shank does not occur when the plug is unthreaded from an engaged cylinder head opening, and the gasket remains captive on removal.