It's associated with the block cooling system but is plugged closed on all (most?) L6 engines. Mine has some mineral build up on the innermost threads and I want to chase it out. A plug I have that fits the hole is, I think, 1/2" BSP but I could sure use confirmation on that.

Datsun Spirit installed a brass pipe thread plug in mine during the build.

You may very well be correct, Yarb, as the plug that was in the engine initially fits into a 1/2"-14 NPT die that I happen to have hanging around. Of course, I don't have a good idea of how BSP differs from NPT so it might be the former.

I think I learned some time ago that whenever Nissan didn't use metric it went with BSP, but that might be a false recollection.

It might be bspt as well.

Edited May 13May 13 by Yarb
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16 hours ago, Seppi72 said:

I think I learned some time ago that whenever Nissan didn't use metric it went with BSP, but that might be a false recollection.

I believe this is the case. Which would make the thread in question a 1/2-14 BPT.

The thread differences between the 1/2-14 NPT and BPT are relatively subtle.... NPT uses 60 degree threads, while BPT uses 55 degree "Whitworth" style threads. So while the pitch is the same, there are some minor differences in the shape of the peaks and valleys.

That said, if you run an NPT fitting in there nice and tight with some thread sealant, it'll probably seal up just fine. Not the "correct" solution, but happens a lot.

Yeah, I studied up on NPT cf. BSP after I posted this thread and learned about the angle and shape differences (and, to my horror, about BSPP). I plan to buy a 1/2"-14 NPT tap and re-cut the hole to (1) clear out the remaining mineral deposit and (2) for a better fit. But first I'll trial that process on some piece of scrap that I might be able to get from a local plumber.

Solid plan.