On a Yanmar it is probably BSPT thread. I don't know why but I only see BSPT threads on Japanese stuff. The 2 most common tapered pipe threads are NPT and BSPT. On a Zerk fitting if the thread is a tapered pipe thread it is most likely gonna be a 1/8 pipe thread. NPT, Natonal Pipe Tapered Thread, which is what you would see in America, will have a 60 degree thread form and for 1/8 pipe will have 27 threads per inch. BSPT, British Standard Pipe Tapered, will have a 55 degree thread form and 28 threads per inch. As you can see the threads are very close. For a Zerk fitting I don't really think using a BSPT in an NPT hole or the reverse will make any difference. I have done it more than once. My reasoning goes like this: The threads on Zerk fittings are usually quite soft or quite hard. So the fitting will either conform to the tapped hole or will make the tapped hole conform to the fitting. In either case you end up with a fitting that will not let grease out or water in. Of course it is best to use the proper thread. But if you can't find the proper thread then use some thread dope or Teflon tape and install the Zerk. This is just my opinion and is worth what you paid. Even though I have been doing this for years it is technically bad practice and I only do this for Zerks. I understand that it is bad practice and if I was doing a job for a customer I would insist on using the proper thread. But for my stuff I have interchanged threads and never have had a problem. I don't really like doing it but sometimes I have had to make do.
Eric