This adapter was specifically designed to extend a generic spark plug tool for use with a Honda XR650L motorcycle. The generic spark plug tool has an 18mm side and a 21mm side. The 18mm fits the XR650L spark plugs, but the tool isn't long enough to be usable. The adapter fits in the 21mm side and attaches directly to a 1/2" socket wrench drive. I successfully removed, installed, and torqued my spark plug to 20 ft lbs with this adapter. Hope someone else enjoys it as much as I do!