Because they're designed like a micrometer, you insert our Pro Pushrod Length Checker in place of a pushrod, and unscrew it until the correct length and geometry is achieved. You then read the height right from the etched markings on the side of the tool, with each revolution equal to .050". This tool makes it less expensive and much faster than any other method used for this critical calculation. If you're not sure what length you need, consult the chart on the inside back cover.