Abstract: An improved hydraulic lash adjuster including a cup-shaped adjuster body, and a plunger assembly disposed within the adjuster body. The plunger assembly includes a stepped axial bore extending from a body inner end to a central oil passage opening onto a hemispherical pivot head. A valve seat for a check valve is disposed against the step, defining a low-pressure oil chamber in the axial bore. A check ball and lash adjustment spring are disposed against the seat in a high-pressure chamber formed between the valve seat and adjuster body. The body of the plunger assembly may be formed by inexpensive processes to a length equivalent to that of a prior art one-piece plunger to resist torsional side loads which may be imposed on the HLA in use. The separately formed valve seat may also be used in other hydraulic lash adjusters such as incorporated in hydraulic valve lifters.