Stroke Adjuster: These are modified PRod parts that will perfectly fit your Discovery. When doing so it will give you a stroke adjuster that will help you tune your Disco for the best possible efficiency. Being able to adjust the stroke length will help you get the power setting you are looking for with the most efficient use of your air supply. The kit includes a new modified hammer that incorporates the stroke adjuster Spring & Cocking Pin are not included as you will use the factory spring & pin. Or you can upgrade to a new Medium spring.