The diameter of the pin opening, point "A", should be measured at the point where the top of the dust boot will sit. It is important to select a size that is slightly smaller than the measurement you obtained. The elasticity of the polyurethane will allow the dust boot to stretch and provide a tight fit on the stud. The diameter of the base, point "B", should be measured where the bottom of the dust boot will sit on the base. Again, it is important to choose a dust boot that is slightly smaller than your measurement. If you choose a dust boot that is even slightly to large, a gap will be created that will allow the grease to leak out and dust to sneak in.