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To use this ingenious tool from Ferry's you must first measure the diameters of both the valve stem and rocker arm's roller tip. Then, by splitting those measurements and subtracting one from the other, you'll determine where your roller tip should be at mid-lift. Then it's a simple matter of installing the correct-length pushrod and rolling the motor over to half-lift to check the distance with Ferry's tool. Our measurements were within 0.010 inch, which is good enough for about 95 percent of all engines.