Diaphragm pump NPT fittings


Hi all, I’ve just bought a Shurflo 100psi pump and needed some 3/8 NPT male fittings. The supplier said all BSP and NPT threads are the same up 1". I trusted them and went ahead. Using a wrench I can get these fittings in about half way and it’s REALLY tight. Is this to be expected with NPT?


Hello here’s some info I’m copying from google but I work at a ace hardware and run into this issue a few times.

I found this to explain things more proper but conveying the same info I know.

“BSPT equivalent is NPT and BSPP’s equivalent is NPS. Never swap threads if it is a high pressure application. NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most sizes have a different pitch.”


BSPT (British Standard Pipe Thread) is similar to NPT except for one important difference. The angle across the flanks of threads (if you sliced the fitting in half long-ways and measured the angle from root to crest to root) is 55 degrees instead of 60 degrees as it is for NPT.

Your best options here are-
If both items are metallic you can always “heat weld” them together but this you can not use thread tape as it would melt /exhaust fumes. But you heat both metal objects to nearly red. Of course using tools / leather / furnace gloves begin to attach both piece and twist, when it reaches the point it can’t easily tighten and you force tighten it, it begins to “weld” the pieces together.

If it is plastic to metal OR plastic to plastic -

Add thread tape and force tighten pass the “hard to tighten stage” using a table vice or something similar and just crank on it safely and allow the materials to cross thread and pray the 100psi doesn’t blow it.

Or similar heat weld as mentioned above… it’s harder on plastic to plastic but you heat up (don’t use direct flame as this would melt the plastic near instant) the male end and tighten as necessary (sometimes you can get the plastic hot add some thread tape that sorta melts instead in, then tighten female on.

Metal to plastic, heat um the male end and attach female and thisbbasically melts the plastic to form a seal.


I don’t understand, you’re trying to put a bsp female onto a mnpt? If so, then yeah, it’ll only go halfway up. bsp is tapered. A male bsp should thread on a fnpt, but you might want some plumber tape on there.

(yes, I’m assuming the genders of pipe fittings because unlike social nutbars, pipe threads only come in two genders)


Your wrong actually there are plenty of pipe fitting that have an external thread of 3/4 with internal thread of 1/2.

Also an aerator for your sink faucet can also have external and internal threads known as dual thread universal…
we call these trans threaded!

(Mic drop)


Nope, those are hermaphrodites. Very rare and cannot reproduce.

A trans fitting would be one that looks female threaded but is in actually male. No such fitting exists.


I guess we are both entitled to our own opinions. But I am very open to the fact the fittings can be whatever they choose to be, they can get cosmelted surgery if they choose to change what they are.


Wikipedia doesn’t lie.


I can give you a straight up answer on what fittings to use if you let me know what you are trying to achieve. I had to convert my Shurflo from 3/8 to handle Rhino Tube which is a metric inside diameter, slightly smaller than the 5/16 hose.


Thanks for all the replies. Well this has turned into a storm on my side… I bought a Shurflo pump from Agratech and it has 3/8 female tapered NPT threads on it. I’m simply trying to connect my pump to ANY hose. With an extra payment they sent me 2 male BSP (to hose barb) connections and said they are 100% Shurflo and will work. One doesn’t even screw in and the other gets half way with a LOT of force. Not to mention they aren’t even tapered to fit inside the female NPT. Chris from Agratech told me he’s been doing this for years and I’m wrong.


Thankfully 3/8" can easily be found at Lowes, Home Depot etc. Take you pump there and find the best match. https://www.lowes.com/pd/B-K-3-8-in-x-3-8-in-Barbed-Barb-x-MIP-Adapter-Fitting/1000157977


You would think right ! Not necessarily true though. I was looking for a 3/8 to 1/2 reducer for my pioe strainer , An Home Depot didn’t have it, nor did they have the 3/8 90 degree 5/8 NPT I was looking for.
So I said screw it sn ordered through Amazon.


I never found any of the plastic NPT fittings that would work quite right going from the 3/8" NPT to the 5/16 hose. The brass fittings (purchased at Menards) aren’t ideal for threading into plastic because the thread pitch isn’t quite the same, but it works and doesn’t leak. The first fitting is a 3/8 to 1/2 with a small o-ring I installed so it seals against the body of the pump outlet. The middle fitting is 1/2" female to standard 5/8 garden hose female. The top fitting is just a 5/8" female hose quick disconnect with the male garden hose thread. The aluminum male garden hose quick connect (purchased WCR) with the 5/16 barb is connected to my hose. The quick disconnect is nice because I can pop the hose right off if I need to service the pump or bring it inside to keep from freezing.