There are some that screw into the plug hole, but you then need to make sure you have the right adapter to fit the threads. Handy if you will be testing compression without an assistant (you don't need to hold the gauge AND kick the bike). If you have an assistant, there is the type with a rubber tip that simply presses up to the plug hole. The nice thing about these is that the tube between the adapter and the gauge is pretty short, giving you a closer-to-truth measurement. The longer tubed setups are still useful as you mainly want to know the RELATIVE compression between the cylinders, not the absolute.
