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Pettis checks the roller tip as it moves across the valve. They use a spring height mic and perform this task on the bench. It simulates peak-lift, mid-lift, and zero-lift. Pettis uses this technique rather than just eyeballing it because he can assign a numerical value to the valve tip movement and plot out the most desirable sweep pattern. The head was machined to get the proper height rather than relying on shims to move the base around. Eliminating the shims helps with valvetrain stability.