18m spark plugs for the Yamato's have become very difficult to find. Here is your option, 14mm to 18mm reducers are available from RPM Marine 206-283-4773, Tom Johnston 517-451-9120 as well as on the internet. However when making the change to using the reducers the heat transfer from the spark plug to the cylinder head is not as efficient and you will want to use one heat range colder spark plug. For example, if you were using a NGK heat range 9 you will want to go to a heat range 10, if you were using a 10 you will want an 11. Here are some of the NGK spark plug numbers, B10ES, B10EV, B10EG, B11ES, B11EV and B11EG