Some questions about fuel line fittings

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dovekie

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I am replacing the diesel line from fuel tank to engine. I plan to use flexible hose (I understand the relevant ISO numbers and the importance of the correct type of hose clips) and put two filters in line. My questions are:
1) When connecting BSP threaded items, should a parallel thread only ever connect to another parallel thread?
2) Does a parallel to parallel connection need PTFE tape or similar?
3) Does a tapered thread to tapered thread connection need PTFE tape?
4) Which is better - taper or parallel
5) What about valves? I can see ball valves, gate valves and tapered plug valves - which is best in a diesel line?
Apologies for my ignorance, but I don't want to get this wrong.

Jon
 
Hi Jon, I personally would go for good quality ball valves, they are quick to operate (quarter turn)and the seal is very positive, make sure that the seal is compatible for diesel. I would also recommend parallel thread cone fittings they do not need any form of sealant.
All the best for 2008
Willie /forums/images/graemlins/smirk.gif
 
The best fitting is obtained by putting a male tapered thread into a female straight. This allows the fitting to be taken up tight with good interference between the threads, for good leak tightness.
When parallel to parallel fittings are used, the male seats at the base of the female, possibly not forming a seal at the bore where the pipe passes through.
It is good practice not to use PTFE tape, or any other sealant, in fuel lines. Liquid PTFE is excellent if you can find it, as it does not release strings of tape and seals incredibly well.
When parallel to parallel threads are used, often the only possibility as chandlers rarely seem to stock tapered, avoid using a sealant downstream of the last filter, as the injector nozzle can then become blocked by debris.
Small ball valves are usually used in fuel lines.
 
Wherever possible on fuel lines use copper or cuniform pipe with compression fittings.(olives etc) The type of fitting you will use on things like filter heads etc will usually be a parallel thread with a taper seat. PTFE tape is not recommended for fuel lines at all and there is no need to use any form of liquid sealant either if the right fittings are used. In most circumstances a parallel thread fitting can be sealed with a copper washer under the head of the nut on say a ball valve using fittings that resolve into a compression fitting on the business end for the pipe. Never be tempted to use fibre washers and make sure the screw does not "bottom out" before the washer is compressed by screwing the fitting in first with no washer present. If you use plastic flexible hose make sure it is clipped back well out of the way of anything hot. Personally I never use it as it goes brittle in time and can crack. Never ever use a taper thread screwed into a parallel fitting. It will not seal properly and could well split the female. In fact I never use tapered thread fittings on fuel lines. Water yes, even gas with PTFE tape (the right kind) but never fuel. Even then a tapered male should only ever be screwed into a tapered female and never overtightened. I concur that ASAP supplies can supply everything you need to do the job properly.
 
AFAIK, the relevant British Standard does not recognise parallel male threads as being suitable for making pressure tight joints, (where the seal is effected by the mating of the threads) , so any joint using one will be of poor quality.Don't forget that diesel fuel is very searching. It is probably worth taking a bit of trouble to avoid the need to use such a thread, perhaps by sourcing components from a more specialised source than the local chandler.
 
Yes they are also a good supplier and looking at their website will tell you that all the fittings they supply for fuel have parallel threads. Where these are screwed into a part they require a copper washer and will not seal on ther own.
 
Joints on fuel lines should never seal on threads anyway. Even a taper to taper thread will not seal well without PTFE which is not compatible with diesel or petrol. All joints should be face sealed using copper washers or be compression type using an olive.
 
1) It is the male thread that is tapered and it usually screws into a parallel female thread. The joint is made between the screw threads and some form of jointing material will be required. That may be a jointing compound that is suitable for the fluid involved or it may be ptfe tape.

2) A parallel to parallel joint will have shoulders which tighten onto some form of joint washer. That may be of any of a whole range of materials. Plain annealed copper, vulcanised fibre, plastic etc etc. Generally the washers should not be reused. Note the joint is not made on the threads so a jointing compound or tape is not used.

3) Never come across tapered to tapered. If they do exist then since the joint will be formed by the threads then a jointing compound or tape will be required.

4) tapered or parallel best? Don't know but probably joints that will be undone and remade periodically will be parallel threaded and a new washer used on reassembly, but see the other joint types mentioned below.

5) Can't really say on the subject of valves. But you have omitted diaphragm valves from your list! Small tapered plug cocks, ball valves or small diaphragm valves will be what you require I would think.

Back to pipe threads be aware that in addition to BSP (British standard pipe) thread which is I think a Whitworth form thread there is an American thread which is very similar. I assume it's a UNC thread form differing in form but not in pitch. To some extent interchangeable though not advisable perhaps. There is also a metric thread. Totally different!

There are other joints that you are likely to come across and use on your installation.

Compression joints in which an olive is clamped onto the outside of a pipe as the fitting is tightened. Jointing compounds are not used on these. They are not suitable for frequent remaking as the copper (or brass) olives harden. They have to be cut off and a new joint made with a new olive, which usually means replacing the whole pipe length.

Conical joints in which the fitting has a conical socket and the pipe end has a fixed (brazed on) fitting with a slightly spherical form that makes a metal to metal joint in the conical socket, and a nut which tightens the joint faces together, No jointing compound should be used on these. These are suitable for periodic dismantling and remaking provided the joint surfaces remain clean and undamaged.

Also joints that are made by forming ball ends or flared ends on the pipe. Often used on car braking systems but suitable for small diameter fuel pipes. Special tools are used to form the ends on the pipe and AFAIK it is special pipe as well. Again it is a metal to metal joint that is suitable for some dismantling and reassembly. Jointing compounds are not used.

I hope that helps a bit. I think you can see that jointing compounds or tape are used for tapered thread joints, joint washers are used for parallel threaded joints with shoulders. Elsewhere neither is used.
 
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It is the male thread that is tapered and it usually screws into a parallel female thread
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Why oh why do people sound off talking such utter bollox about something they know nothing about? As an engineer who ought to know something about screw threads I am not going to argue with a retired chemist but you are so totally wrong that you need to be corrected to avoid misleading others.
Just Google British Standard Taper Pipe Threads Or American Standards if you prefer and you will see both male and female taper threads are standard and I can definitely assure you that male taper threads are NOT designed to fit female parallel threads. Forcing a male taper into a female will split the female if its done up too tight and it is a supreme butchery to force a taper thread into a parallel female one. Just bloody look it up!!! The internet will provide the evidence. You are totally wrong!!!!
 
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talking such utter bollox

[/ QUOTE ] OK I'm am wrong and you are quite right in correcting me BUT THERE IS ABSOLUTELY NO REASON FOR YOU TO DO IT IN SUCH AN OFFENSIVE MANNER.

I suppose after working for nigh on 30 years in an engineering environment I should have learnt what an ill mannered an offensive lot you engineers are! Maybe a few later years with other scientists followed by a few years in academia has clouded my memory. Thanks for refreshing it!
 
Thats fine, glad to remind you that we don't like people talking bollox when some poor sod is asking for advice especially when I had already said the exact opposite. Why did you feel compelled to argue without checking in the first place? Forcing tapered threads into parallel fittings can certainly be done but it is a major cause of failure in cast fittings. I once spent 2 hours in a F8 60 miles off the Portugese coast in a 70 ft yacht looking for an air leak in a fuel line throwing up in the bilge trying to get an engine running again after we had put the fire out because a fitting had split because some pratt had rammed a taper thread into a parallel thread. Your advice, if followed, could result in someone else being in this situation! I make no excuse for being rude. Check before arguing next time and I won't need to be.
 
I quote from Kemp's Engineers' Year Book regarding BS 21. "This standard relates to joints made pressure tight by mating of the threads and includes taper external threads and both taper and parallel internal threads. ( Parallel external threads are not suitable as jointing threads )".
This therefore does not appear to preclude the mating of a parallel internal thread with a taper external thread. If it did, there would be no point in a standard for parallel internal threads. Or is the book wrong?
 
I am trying to keep calm here and give you a logical answer.
Taper threads are designed to be mated with taper threads otherwise, turning your argument on it's head, there would be no point at all having a female taper thread in the first place would there? Taper threads are designed to mate with taper threads, and where they need to be water or gas tight are usually sealed with PTFE. It is possible, but extremely bad practice, to screw a tapered plug into a parallel female. It puts an enormous bursting stress on the first few threads of the female part which distorts it and if it is a brass or even bronze cast fitting can cause it to split. There are exceptions, like malliable iron fittings for gas conduit where it is permissable as the iron fitting will distort to fit the pipe. Here we are talking about marine materials and best practice on fuel lines however which is to use parallel threads and not seal on the threads at all. Also instead of sealing over the full length of the thread as it is designed to do a tapered male in a parallel female will only seal on one or two threads. Your quote from Kemps is therefore out of context and only relates to threads that are made pressure tight by mating on the threads anyway. As I have already stated this is fine for water and gas but bad practice on fuel lines where best practice is a face seal achieved by a copper washer or a compression fitting. Have I now answered your question adequately?
 
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compelled to argue ..... arguing next time

[/ QUOTE ] I was not arguing with you or any one else. My post was a straight reply to the original question.

Must make the point that even tapered threads into tapered threads should not be overtightened as there is still a risk of splitting the female fitting, and it is very easy to over-tighten joints made with ptfe tape.

I have tried a Google search for "British standard taper pipe threads" as you suggested but so far i have not come up with anything particularly useful and nothing stating that tapered threaded components must not be screwed into parallel threaded fittings. One or two specific links would be appreciated, then at least I will have some ammunition to use against the next plumber I come across mixing the two.

I note your arrogance in refusing to moderate or apologise for your continuing use of offensive language. I think you will find it is against the forum rules so perhaps you would prefer the moderators to remove your posts for you!
 
My quote was not out of context. Machinery's handbook says exactly the same thing, adding specifically that external male threads may be mated with parallel internal threads. Your assertion that this is bad practice is clearly not endorsed by British Standards. American taper thread standards follow similar rules. I see no qualifier for material type.
Why do you accept this practice for gas, but condemn it for diesel fuel? Why can't PTFE tape be used with diesel? It is a very inert plastic. If you can show authority for these choices I would be interested.
Any jointing system can be butchered by careless assembly and poor fitting practice, but that is no reason to totally condemn that particular system. I have seen more than a few mangled compression joints, indented copper washers etc etc. Certainly, metal to metal sealing, as you describe, would be my first choice but in the real world, we have to use what we can reasonably acquire. This was the thrust of the earlier posts. There must be many, many thousands of fuel systems containing taper thread joints, and giving entirely satisfactory service.
 
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Why can't PTFE tape be used with diesel? It is a very inert plastic

[/ QUOTE ] While in no way now prepared to take boatmike's side in this i think there is some objection to its use after the filters as little bits can come off and block the injectors. Probably applies to other joining materials as well so perhaps metal to metal joints or parallel threaded joints with copper washers are better choices here.

Just my thoughts, I'm only a retired chemist so don't have a clue really.
 
"Just my thoughts, I'm only a retired chemist so don't have a clue really."

Now, now chaps, deep breaths /forums/images/graemlins/smile.gif You make many valuable contributions, for which I thank you, and as a retired chemist, do you know if alcool a bruler is ordinary ethanol, ie the French version of meths? (apropos the post on spirit stoves?)
 
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do you know if alcool a bruler is ordinary ethanol, ie the French version of meths? (apropos the post on spirit stoves?)

[/ QUOTE ] Just, by chance, finished an explanation of all that on that thread.
 
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