Adding a heater to a Kubota. Metric Threads?

[robstaples]

I'm adding a heater core and the recommendation dedicated coolant ports have plugs. They are close to a 3/8" NPT but I only get 2 turns and they stop hard where the plug goes in all the way. I'm looking for the right thread size rather than deform the brass adapter. It's approx 19 threads per inch and tapered.
This is for a L39tlb and i would guess a common thread size on Kubota engines. Staff at Merrick's were very helpful but couldn't tell from part descriptions.
Does Kubota use metric pipe threads or British Standard pipe tapered? Or something else?
Thanks in advance

 

[KWentling]

Yes, that would be BSP.

 

[rScotty]

Yes, that would be BSP.

Agree. I've seen that problem before. Lots of Japanese designers use BSP. The story I heard is that a shop loaded with machine tools spent the winter in Japan just about the time that the Japanese began to industrialize. Have no idea if that is true, but what you say about it not quite "feeling" right after a couple of turns is diagnostic for trying to fit NPT into a BSP thread - or vice versa. Good on you for noticing. Most don't notice...and the theads are close enough that you will get by with the mismatch just often enough to cause trouble. Study up on the thread specs and you'll see why.

As an aside - for the designing engineer there are a variety of thread & taper choices for making compression connections. Unless it's some sort of laboratory application or extreme duty, then NPT is what most of us favor. but the BSP also makes a decent connection if you have the proper threading equipment and parts. So I can't fault the Japanese designers for choosing that spec....only wish it were farther removed from NPT so that mistakes can't happen.

There is a metric tapered thread as well, but I have not had much luck with it and don't know much of anything about it. I have seen it used on some continental European machinery.
rScotty