By using this tool , the water jacket is pressed down over the o-rings to properly seat the water jacket .This tool threads into the sparkplug hole / tighten the jam nut to push the jacket down . Next install the 3mm bolts and tighten accordingly . This will lessen the possibility of stripping the small bolt holes . We have been using this style of tool for years . Very handy . Comes with bolt / jam nut / 2 steel washers / 1 nylon washer and a protective sleeve for storage .