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prephil (Mechanical) (OP)
8 Feb 07 9:31
Hi, I'm looking for tables that convert NPT or BSP inches to standard imperial inches. Does anyone know of such a table?

God bless,

JLSeagull (Electrical)
8 Feb 07 17:00
Are you looking for nominal pipe thread or nominal pipe size?
prephil (Mechanical) (OP)
9 Feb 07 3:14
National Pipe Thread
TBP (Mechanical)
9 Feb 07 9:52
prephil - Your initial question reads as though you're looking to compare National Pipe Taper thread to British
Standard Pipe size. Are you asking if it's possible to cut NPT threads on pipe that is British dimensions?
prephil (Mechanical) (OP)
9 Feb 07 19:52
No I just want to convert NPT to imperial inches or BSP to imperial inches. I'm a little confused as the lads at work are telling me BSP inches are different to imperical inches. Am I right in thinking this or is a 3/4" NPT equal to a 3/4" imperial and ditto for a BSP?
NozzleTwister (Mechanical)
10 Feb 07 12:01

You might try the following links covering different thread standards.

Houston, Texas

TBP (Mechanical)
10 Feb 07 12:49
prephil - You're also getting some terms crossed. "NPT" isn't a pipe size, it's a thread designation.

In the early days of manufacturing, pipe was designated from the ID. It was determined (pretty quickly) that the wall thicknesses that were being produced were big-time overkill. In order to reduce the wall thickness, the ID was increased, rather than the OD being  decreased. I suspect the reasoning was that there was already quite a large installed base of piping. To increase the OD of pipe would mean all of the existing fittings, valves, dies, connected equipment in plants, locomotives, ships etc would be obsolete overnight. Not happening. This is why there isn't one dimension on 3/4" pipe - of any wall thickness - that is 3/4". This is true up to 12" pipe. For that pipe size and below, everything is nominal. I suspect something similar is true of British pipe. Additionally, ANSI pipe and BSP are not the same dimension.

NPT thread that is the standard in the US & Canada was originally called "Briggs thread", after it's inventor. Interestingly, Briggs was an Englishman who returned to the UK, and I believe, developed the British standard pipe thread there. Both the US and UK pipe & threads are dimensionally close to each other, but yet are NOT the same. And trust me, while they are CLOSE, they are very much NOT close enough to work together. Ask me how I know that :(
Helpful Member!  11echo (Petroleum)
14 Feb 07 16:49
Well let’s break this down and see if we can't cough up an answer:
N.P.T. = National Pipe thread Tapered, this is a piping thread based on 60 Deg. angle cut thread on a tapered slope so when you tight the N.P.T. threaded connections they jam together to make a tight seal (normally)

B.S.P. = British Standard Parallel threads are a piping thread that the British use on their piping systems, that is based on a 55 Deg. cut thread that is cut parallel on the pipe so that as you thread this connection down it seals at the end of the pipe.
Now there is a N.P.S. = National Pipe thread Straight, now this is a parallel cut thread but it is based on a 60 Deg. angle and is use on electrical conduit type fittings.

SO if your trying to use N.P.T. or N.P.S. threaded fitting with B.P.S. threaded fitting, it CAN'T be done because of the type of threads 60 Deg. and 55 Deg. cuts. They are not compatible. HOPE this helps!
prephil (Mechanical) (OP)
15 Feb 07 17:40
Those links you pasted were perfect.
Before I thought 1"NPT was  1" OD but it's 1.315
Everything is a lot clearer now. God bless.

TBP - You've confuse me a little. When you say "there isn't one dimension on 3/4" pipe - of any wall thickness - that is 3/4". Do you mean that the 3/4" will refer to the OD and the ID can vary among different pipes?
TBP (Mechanical)
16 Feb 07 9:54
3/4" pipe has an OD of 1.050". Sch 40 3/4" pipe has an ID of 0.824", while sch 160 has an ID of 0.612", and XXS has an ID of 0.434". The ID for sch 80 is actually pretty close to 3/4" - it's 0.742". The OD remains unchanged regardless of wall thickness. If the OD changed with the wall thickness you'd need a different set of dies and fittings for every variation. That would be confusing, expensive, and unnecessary.

Once you go 14" & over pipe, and the OD actually matches the pipe size, various wall thickness still only affect the ID of the pipe. You will get noticeably lower flow capacity from sch 160 pipe vs sch 40, even though they're the same OD.

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