NPT or NPS

[Rivergator]

Mercruiser part: 22-42683 3/4x3/4 brass hose barb.
Does anybody know what thread this is? I know it is a 3/4"-14 but I need to know whether it is a NPT (tapered) or NPS (straight). The reason for my question is I need to replace the barb and I want to chase (clean) the female thread in the bell housing first.

 

[Bt Doctur]

I would venture a guess it`s NPT

 

[Bondo]

Ayuh,..... Agreed,.....

 

[achris]

The only straight threads in the bell housing are the hinge pins and the shift cable...

 

[Rivergator]

Thanks you all for the info. I love it when everybody is on the same page. A unanimous vote for NPT. If you are a anal stickler like me I came across the official Mercruiser specs for the barb. it says NPTF (tapered fuel) which seems a little overkill. I mean nothing but cold cooling water runs through it, but never the less NPT is totally correct. Now I can order the correct tap. Thanks again.

 

[todhunter]

NPTF typically means "National Pipe Thread, Female", indicating the threads in the bellhousing are female threads.

 

[Rivergator]

todhunter, there is so much info floating around and so many variations but you may very well be right. I don't know. I read somewhere that it stands for "fuel". But I admit, I don't see the difference. NPT, I guess is NPT. The only difference could be in the sealant that one applies in a fuel application. On the other hand the specs are strictly dealing with the brass barb, which is male and at this point has no idea that it is going to be screwed into the bell housing which is female, yes. But you made an interesting point.

 

[Rivergator]

I did some more research and the F does stand for "fuel" and my theorizing about the sealant was false. A NPTF thread is pitched in such a way, that when properly torqued will seal the connection without any sealant, which is important in a fuel application, because fuel will ultimately attack any sealant.

 

[achris]

I did some more research and the F does stand for "fuel" and my theorizing about the sealant was false. A NPTF thread is pitched in such a way, that when properly torqued will seal the connection without any sealant, which is important in a fuel application, because fuel will ultimately attack any sealant.

You use NTFE thread tape or paste as a lubricant, not a sealer (ever!). Some books will say not to use thread tape, particularly on fittings used in hydraulic circuits, only for the reason that if some of the tape overlaps the end of the fitting, it can come loose and foul very fine hydraulic valves. In a fuel setting, I have use both with no ill-effects. Just be VERY careful to not get any tape off the end of the threaded fitting.

Chris..........