This tool from KD Tools is designed to re-form the threads in a cylinder head to enable proper installation of the spark plug. It also is useful in cleaning out the threads of carboned up heads to ensure correct thread engagement for better heat transfer.
It features an extra-long neck for access to recessed spark plug cavities. The groove and O-ring holds tool in the socket. Use with 3/8_ square drive tool or a 5/8" socket.