Any and all engine builders, whether building a bone stock motor or a high performance machine, should have this tool in their tool box.  Finding combustion chamber cc's is a critical part of determining compression ration and setting up your engine build.  To use, simply install a spark plug in the head, coat the outer edge of the disc with Vasoline, and press it firmly into the cylinder head.  Fill the syringe with water, and inject it into the hole in the middle of the sealing disc until the camber fills up and no longer has air bubbles.