Thread sizes

[pcatterall]

I have been trying to get to grips with thread and fitting sizes related to fuel pipe fittings.
I wonder if others have struggled and if there is a quick clear guide that can be recommended.
I now realise that a lot of fittings are decribed by their pipe size British Standard Pipe ( BSP)
and then these may have the suffix T (taper, where the diameter accross the threads reduces) or P ( parallel where the diameter across the threads remains constant).
I am still not sure about the application for these different sizes.
There also seems to be a fine thread NTPF and there seems to a system where the diameter of the threads is alluded to rather than given.
As an example I had bought a fuel pump and needed a hose tail to fit it. The tail part was easy 8mm (5/16" ) diameter. I can see that the threads are fine so I guess that NTPF is required. The thread diameter required measures at 3/8" but there doesn't appear to be that size anywhere.
Back to my question..... is there a recommended guide to help sort out the sizes and if possible the range of fittings available?
It will be much appreciated!!

 

[theoldsalt]

For thread sizes this may help.

http://www.pipefittingsdirect.co.uk/contents/en-uk/BSP Identification.pdf

 

[VicS]

1/8" BSP has a major diameter of 0.383 " ... that is not far off 3/8" . At 28 tpi it is a pretty fine thread.

I dont know if there is a fine BSP thread.


Tapered threads seal on the thread with the aid of some tape or jointing compound.

Parallel threads should tighten onto a joint washer.

Common plumbing bodge, which is not suitable for your fuel system, is a tapered male thread into a parallel female.

There are also American pipe threads, some of which are very close to BSP, as well as metric threads.

"Zeus" data booklet is very useful but there are loads of on line sources of info

 

[pcatterall]

Thanks, theoldsalt. That chart is very useful.
So for my 3/8" threaded diameter fitting I have to subtract 1/4" and order it as a 1/8th item ( is that just to confuse us all!!) I have copied that chart and will keep it in my caliper box!!
I also note the different thread pitches and cut angles on parallel and taper threads and guess it would be easy to damage items by trying to use the wrong thread even though it may seem to 'start' easily.

 

[boatmike]

1/8" BSP has a major diameter of 0.383 " ... that is not far off 3/8" . At 28 tpi it is a pretty fine thread.

I dont know if there is a fine BSP thread.


Tapered threads seal on the thread with the aid of some tape or jointing compound.

Parallel threads should tighten onto a joint washer.

Common plumbing bodge, which is not suitable for your fuel system, is a tapered male thread into a parallel female.

There are also American pipe threads, some of which are very close to BSP, as well as metric threads.

"Zeus" data booklet is very useful but there are loads of on line sources of info

Second all that but for a very simple recognition of BSP sizes go to the ASAP Supplies website. Also although one of the pictures shown on one of your other threads shows a tapered thread screwed into a parallel fitting and the poster has said it was assembled with thread sealant. This is NOT recommended for fuel as Vic has said. As I said earlier the male part should either seat on a taper seal face at the bottom of the thread or you should use a parallel face to face fitting with a copper washer which should NOT bottom out. I recognise that if you are not used to all the alternatives available for water, oil, fuel etc it can indeed be confusing but if you follow the simple rule of never using PTFE tape or thread sealant on diesel lines you won't go far wrong.

 

[nigelmercier]

I have been trying to get to grips with thread and fitting sizes related to fuel pipe fittings...

The thing you have to remember is that this is pipe, so the older standards are based on the internal diameter, this being the important consideration. The standards predate modern copper fittings, and are based on iron pipe-work which had a wall thickness of ⅛" hence the ¼" difference between the pipe size and the OD.

"Zeus" data booklet is very useful...

I can't believe this is still in print, I've had it for about 30 years. Very handy indeed.

 

[pcatterall]

Thanks also VicS.
Yes! your example confirms the rule of 'taking 1/4" off' the measured thread diameter.
I also note from the table that when you drop 1/2"(minus the 1/4) to 3/8" ( minus the 1/4) the threads per inch jumps from 19 to 28 so for my idea that I needed an NTPF thread may be rubbish. What I will probably see my item listed as will be a BSP 1/8" 5/16" ( 8mm) hosetail...... simples!!
Thanks

PS and thanks Nigel... taking off the 1/4" ( for pipe wall thickness) now makes sense!

 

[scottie]

Watch out for sae? Fittings used in some lift pumps and cav type filters you can butcher a 1/4 bsp into it but it will leak

BSP used to be known as gas sizes as it was base on the bore of a steel gas pipe in inches so plus 1/4 would make sense

I was always told that using any thing to seal a fuel pipe was NONO!

 

[jdc]

Racor threads

... one of the pictures shown on one of your other threads shows a tapered thread screwed into a parallel fitting and the poster has said it was assembled with thread sealant. This is NOT recommended for fuel as Vic has said. As I said earlier the male part should either seat on a taper seal face at the bottom of the thread or you should use a parallel face to face fitting with a copper washer which should NOT bottom out. I recognise that if you are not used to all the alternatives available for water, oil, fuel etc it can indeed be confusing but if you follow the simple rule of never using PTFE tape or thread sealant on diesel lines you won't go far wrong.

That was me I think, however I'm not using parallel into tapered threads. The Racor filter has a 1/4" NPTF internal thread, so the face-face fitting with Cu washer is not appropriate, and both the male and female fittings I use are tapered.

The F in NPTF is for Fuel: wiki has this to say:

"National Pipe Taper Fuel (NPTF) also called Dryseal American National Standard Taper Pipe Thread, defined by ANSI B1.20.3, designed to provide a more leak-free seal without the use of teflon tape or other sealant compound. NPTF threads are the same basic shape but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path."

So thank you for pointing out that adding a drop of loctite sealant isn't good practice, or is at least unnecessary with NPTF - although I'd never use PTFE tape, I hadn't realised that any thread sealant was bad.

 

[boguing]

There is a very handy android ap - Thread Pitch lite - that you'll like if you have a suitable device. The backlighting from the screen makes identifying so easy. Much more so than proper thread guages.

 

[pcatterall]

All very interesting!! I will have a look at that ASAP web site.
A question now arises ( sorry) .... if I want to screw an elbow into say a filter head and need to get it tight but at a certain angle what is the best way to seal it? With wate rI remember we could use tape but I can see this is an issue with fuel. JDC ( I think) had suggested that a drop of loktite would work but is this also a no no !!

 

[scottie]

To fit a pipe at a specific angle try a banjo and banjo bolt