BSP vs NPT Threads
The first thing to understand about water filter port adapter sizes is that, as with all plumbing, there are two main standards/types.
NPT is the American Standard - mainly used in the USA and a few other countries. (NPT stands for “National Pipe Thread”).
BSP is the British Standard widely used around the world. BSP stands for “British Standard Pipe”
While BSP is more widespread the chances are that the water filter that’s ended up in your property was made in China.
This is because the Americans led the world in domestic water filtration. Chinese factories often used to copy American water filters (sometimes down to the slightest blemish).
Even if they designed their own, similar filter housings, they could sell these for a fraction of what US or European companies had to charge so Chinese-made filter housings came to dominate some parts of the domestic market. This is why it’s possible that, even if it's supposedly British or European, your water filter has NPT female ports.
Bear this in mind if your male adapter has broken and you're looking to replace it.
What's the difference between BSP and NPT
NPT and BSP threads are not compatible due to:
1) The differences in their “thread forms”: NPT has flattened Peaks/crests and valleys/roots while BSP has rounded peaks and valleys.
2) The differences in their “thread flank angle”: NPT threads always have a 60° thread flank angle, while BSP has a 55° angle.
3) Most sizes have a different “thread pitch” - ie the distance between threads.
How to Feel the Difference Yourself between BSP and NPT threads
When inserting the male of one standard into the other standard’s female, it should feel smooth/easy for the first couple of hand turns. If it jars ie grates slightly then its most likely a BSP trying to fit into an NPT(or vice versa).
Metric or Imperial?
Confusingly the US / NPT refers to Imperial aka “fractional” sizes eg ¼ inch while the BSP sizes will quote either Imperial or Metric sizes.
Can I Mix NPT and BSP parts?
In water filtration plumbing the difference between some NPT and BSP parts can be very marginal and you can sometimes get away with using one with the other - eg a male NPT on a female BSP part - but only IF they are PLASTIC.
Definitely, don’t do this if they are metal. (Unlike plastic, the threads won’t bend together with pressure).
But you'll need to use a lot of PTFE (plumbers tape) or other sealant. And don't be surprised if there is a very small ongoing leak/weeping.
The true professional would say Never mix different standards... and certainly never if it is a high-pressure application.
If you are looking at a small gauge plastic-to-plastic part, and you do decide to risk it, you might only get very small leaks which you can usually rectify by using excessive PTFE (plumbers tape) or other sealant).
While you might be able to use a BSP with an NPT part please note any replacement part you order will be AT YOUR OWN RISK.
You must observe it carefully for sufficient time (several minutes and regular checks afterwards for several days).
If it doesn’t work ie the part leaks / weeps slightly, we’ll know which standard it is and should be able to send you the alternative / correct part.
If you have any doubts simply buy a thread gauge - they can cost less than £10 online - and use the above BSP vs NPT guide to find out exactly what you need.
See other BSP / NPT Codes
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BSP vs NPT Types
Some General Terms Used for Screw Threads
Replacing Housing Adapters / Ports
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Chart of Tube Size Adapters Combinations