Honda Washer (6mm) 90512-921-010 - Overview. The Honda Washer (6mm) 90512-921-010 is a genuine OEM flat washer designed specifically for Honda marine engine systems. This essential hardware supports the secure fitting and mounting of various engine components and rigging accessories, ensuring reliable operation of your boat's mechanical systems. Designed to meet exact factory specifications, it integrates seamlessly with existing hardware to maintain optimal function and durability.. Key Feature