These Hydra-Solid tappets have the same "Race Proven" quality features as the Fat Axle tappets but as an extra feature Hydra-Solid tappets give you the combination of the best of the hydraulic tappet and the solid tappet. They act like a normal hydraulic tappet from zero to 5.500 rpm, but as soon as your engine exceeds the 5.500 rpm the tappet acts as a solid tappet, adding some 3 to 6 useable horsepower. These tappets are precision machined, than heat treated and finally ground to less than .0002" total readings at a 16 finish. They are called "Fat Axle" because they feature an extra thick roller pin and extra thick rollers for increased roll strength and will not deform, even when used with extreme high lift cams. Strongly recommended when using Performance cams as they give zero valve lash, and zero hydraulic collapse. These tappets are extremely competitively priced but still are the best money can buy. Fit in conjunction with adjustable pushrods in Evolution Big Twin 1984-1999, and Evolution Sportster 1986-1990. Tappets have stock .8425" diameter and are sold each.