I put a gauge on temporarily when I suspected the oil sender was bad. (it was). rScotty here had said Yanmar used BSP (British) threads, similar to USP but less deep groves and more taper. Or something like that.
Experimenting with an assortment of USP fittings etc about half of them were to fat to start threading into the block. I ended up using a brass fitting assuming that would do the least damage to the block's threads, then a USP tee and gauge.
Post where I used that photo. Also, I started that thread with measurements of normal temperature at head, block, etc that some might find interesting.
ARGHHH. I'm a machinist, so maybe that's why the above post bugs me. Or maybe it's because the wrong designation is used for the threads, a designation that doesn't even exist for the threads being discussed. And folks looking for fittings might get confused. The author using USP must mean NPT and BSP to mean BSPT. So I will try to clarify things a little bit.
NPT stands for National Pipe Tapered. Which is short for National Pipe Threads Tapered. Of course the nation in National is the USA.
BSPT stands for British Standard Pipe Tapered.
Of course there are many other pipe thread standards for each nation with different designations. But we are talking here about tapered pipe threads.
Both the British and USA threads have the same taper per foot, which is 3/4 inch per foot. The angle of each thread is different however. USA threads have a 60 degree angle while British threads have a 55 degree angle. They also have different thread pitches, the number of threads per inch, for the smaller threads, which are the most common on the equipment we use. 1/8", 1/4", and 3/8" are the most common threads on the relatively smaller machines we use. The British threads are finer by one thread than USA threads. 1/8-28 British vs 1/8-27 USA, 1/4-19 and 3/8-19 British vs 1/4-18 and 3/8-18 USA. This works out to .0013" and .0029" difference in pitch respectively. There are also differences in the crests and roots of the threads but they matter little. Since the threads are interference threads, which they need to be in order to seal, slight differences in pitch and thread angle matter little with short thread engagement and if the threads are able to deform so that they conform and seal.
Though it is best to have several threads engaged for mechanical strength and for reliable sealing a person could use NPT and BSPT threads together in a pinch, say for checking oil pressure. To do this the thread in the machine should be harder than the thread on the gauge or similar that is threaded into the machine. For example, a cast iron engine block and a brass pipe screwed into the engine block. Teflon tape or some other type of sealant and lubricant should be used. And the pressures should be low. Like less than 200 PSI. Don't try to measure 1000 PSI with only a thread or two engaged because things might get exciting.
Finally, after the long screed above, simplified, NPT=American Tapered Pipe and BSPT=British Tapered Pipe.
Eric
