I have just acquired a atco lawn mower standard approx age 1923 it has a cast iron cylinder and head all in one. I was wondering what the correct spark plug is for it , it came with an NGK AP6FS, it does not have a washer on it and it is tapered to seal against the head,also the thread on the plug doesn’t look long enough for the thread in the head.
Can anyone tell me the correct plug and number please .
Malcolm