A hydraulic lash adjusting tappet (10) for use in engine valve gear of the direct-acting type have one end of the tappet contacting the end (26) of the combustion chamber valve stem (22) and the other end contacting the camshaft lobe (16). The tappet has a body (34) formed with a tubular wall portion (36) having one end thereof closed by an end wall (38) and with a tubular hub (40) therewithin formed integrally with the end wall and extending axially therefrom. A lash adjuster assembly (44) is slidingly received in the tubular hub which defines a reaction surface (50) remote from the cam face reaction surface (18) defined by the end wall. A seal (88) defines an expansible closed fluid reservoir (114) in combination with the body and includes a compliant diaphragm (90) carried about the outer circumferential portion (92) thereof by said body and a sleeve cap (52) insert-molded within the inner circumferential portion (110) of the diaphragm to provide a central portion (104) wear surface interposed, in application, between the lash adjuster reaction surface and the valve stem.