Abstract: A method of cementing oil well casings and a cementing tool used when performing the method are provided. A novel top cementing plug functions in a conventional manner during proper oil well casing cementing operations, but if inserted into the oil well casing in error, allows cement to be pumped through it. The top plug includes an outer, hard rubber casing having vertically spaced fins formed about its circumference. A central passage is provided through the outer casing and receives a cylindrical, metal sleeve. In one embodiment, a bore is formed in a portion of the metal sleeve to define a shoulder within the sleeve. The bore receives a cylindrical housing which carries a spring loaded, ball valve mechanism. The valve mechanism includes an annular spring retainer which rests on the shoulder. One end of a helical spring within the housing contacts the retainer while the other end of the spring biases a ball against an annular flange at the top of the housing to seal the passage.