13/16" (21mm) Spark Plug Wrench/Socket.
Commonly used for Honda engines such as: GX100, GX110, GX120, GX140, GX160, GX200, GX240, GX270, GX340, GX390, GC160, GC190, GCV160, GCV170, and GCV190.
May work for many other small engines using a spark plug with a 13/16" (21mm) hex head.