A double-effect distribution sequential valve shaft assembly for use within a cylinder head cover of internal combustion engines or other valve-type distribution systems, includes a rotating valve shaft with openings from side to side in the vertical plane and a jacket supporting the valve shaft for rotation therein. An opening in the valve shaft communicates through diametrically opposed openings in the jacket with the combustion chamber twice during every turn of the valve shaft to regulate the flow of gases into and out of the combustion chamber. The jacket is provided with a series of labyrinths etched into an inner surface thereof to create a barrier to gases flowing out of the combustion chamber during the compression and explosion strokes. The jacket is also provided with a plurality of circumferential grooves formed in the inner surface thereof to form a high pressure fluid seal when lubricating oil is introduced therein under pressure. The sequential valve shaft thus has a very high closing efficiency and heat transfer properties in all phases of operation.