Harold W. Evans Sr., 85, a retired civilian official of the old Army Ordnance Corps who helped design armored vehicles used in World War II, died of cardiac arrest Sunday at the Oak Springs Nursing Home in Warrenton, Va.

At the beginning of World War II, Mr. Evans was assigned to the Tank Automotive Command in Detroit, where he served as project officer for the M3 through M5 light tank series. He also helped design armored cars and antiaircraft-gun mounts widely used on trucks during World War II.

Mr. Evans joined the Army Department following active duty in World War I as an officer in the Motor Transport Corps. He retired in 1958 as chief automotive engineer in the Research and Development Division of the Army Ordnance Corps.

He was a member of the American Ordnance Association and had been chairman of the Society of Automotive Engineers.

A native of Wilkes Barre, Pa., Mr. Evans had lived in Takoma Park since 1928. He belonged to Trinity Episcopal Church in Takoma Park and was a 50-year member of Takoma Park Lodge No. 29 of the Masons.

His wife of 61 years, Mary Elizabeth Evans, died in 1978.

Mr. Evans is survived by two sons, Kenneth J., of Acton, Mass., and retired Marine Corps Col. Harold W. Jr., of Warrenton, a sister, Mildred Davis, of Sun Lakes, Ariz.; six grandchildren, and three great-grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Trinity Episcopal Church Memorial Fund. CAPTION: Picture, HAROLD W. EVANS SR.