Their device rolls on top of the rocker near the valve and hooks under the rocker at the adjuster. Super simple. The tool has instructions for adjustments to make them work on different rockers. The instructions say to adjust the tool to match bench testing but that isn't necessary as all we are usually looking for, on the head, are weak differences. Tool works on my single shaft, T&D's and Harland Sharp rockers. You could probably make something flat to get them to work on stock rockers.