Variable duration hydraulic valve tappet

Claim 1

In a hydraulic valve lifter having a hollow cylindrical body with a closed lower end for engagement with a cam; a hollow cylindrical plunger axially slidable in said body and enclosing a pressure chamber in the lower end of the body; an external collecting channel on said body to receive oil from a pressurized source; a port opening into said body from said channel; a port in said plunger communicating with said first mentioned port to admit oil into the plunger; and a one-way valve in said plunger opening into said pressure chamber: an improvement comprising: at least one restricted oil bleed passageway providing, at all points of operation of the hydraulic valve lifter, continuous communication between said pressure chamber and said collecting channel; said oil bleed passageway being substantially dimensionally independent of temperature induced volumetric changes in said valve lifter; the cross sectional area of at least a portion of said bleed passageway being insufficient to sustain oil bleed beyond a predetermined flow rate at high engine speed.