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To accurately simulate valve behavior in an engine employing a Hydraulic Lash Adjuster (HLA), it is necessary to reflect in the simulation model the dynamic characteristics of the HLA when feeding aerated engine oil. Traditionally HLA behavior was measured by agitating oil to aerate it inside an actual engine. With this technique, however, the aeration rate cannot be controlled to allow an examination of the relationship between aeration rate and HLA behavior. This study aims to establish a technique for obtaining HLA dynamic characteristics when oil contains air. We created an aeration device that allows free control of the oil aeration rate and, by applying excitation input to the plunger, revealed the relationship between oil aeration rate and the changes in HLA stiffness. Because of the small clearance between the HLA plunger and its housing, it is difficult for air to escape from low-temperature oil inside the HLA high-pressure chamber, which greatly affects HLA behavior.