You no longer need to fight or snake a flexible rubber hose blindly through the valve cover and into the cylinder head to hook up a compression or leak down gauge on the 911 engines (or other engines with remote spark plugs). Simply insert this unique tool into the spark plug hole of the cylinder head and attach your gauge. Precisely machined from high quality stainless steel or hard-coated aluminum. Available in two sizes based on spark plug thread/diameter. Sold separately.