99004-21001 Box (P21) Wrench - Overview. The 99004-21001 Box (P21) Wrench is a specialized tool designed to support maintenance and repair tasks within Honda marine engine systems. Specifically engineered for use with Honda marine engines and other Honda equipment, this 21mm box wrench is essential for spark plug installation and removal, ensuring your equipment remains reliable and operating at peak performance. Precision-crafted to fit factory specifications and tolerances, it seamlessly integ