Sealing joints on diesel fuel lines?

stav

stav

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Hi all,

I am working through the list of jobs and nearly next is to fit a new fuel filter and an out of engine compartment 1/4 turn valve. I have tried searching but too many other results, so what are the does and don'ts of ensuring the new pipe work will be fuel and air tight.

From the tank to the valve will be 8mm copper with compression and small BSP/UNF fittings. Out of the filter will be 8mm reinforced fuel hose (already there and one end is completely inaccessible).

I understand PTFE tape should not be used but what else can be used to ensure a sealed system? I have some blue hylomar sealant and was thinking of using some of this sparingly to ensure none gets in to the fuel line. I was wondering about some type of liquid thread lock/seal?

Any advice welcome as I do have a leak free system and want to achieve the same but with the extra racor spin on filter and emergency valve and I am keen to do this before trying to rebuild the injectors and fitting them.

Thanks in advance for any thoughts/advice.
 
Taper threaded fittings will often require sealant.

However, the best solution is to use no sealant whatsoever, especially on compression fittings. If a joint leaks when tightened, replace the olive. If it still leaks replace the fitting.

Using sealants on compression fittings will increase the risk of leaks in the future and an air leak when you need full power would be very boring.
 
Last edited:
>You can get sealant designed for fuel lines, plumbers should sell it for sealing domestic oil fired boiler lines.

You should also be able to buy it at Halfords, only use sealant designed for diesel.
 
I would endorse what's been said above, NO SEALANT on copper compression fittings, just PTFE on the taper threads.

I replumbed all my fuel lines last year and fitted a filter changeover system, so lots of valves, pipes, and joints.

No leaks.
 
I use a tiny drop of Clessetite on all the copper heater tube fittings I use, never had a leak, either air or diesel its non setting and you cen even pressure test LPG lines straight after assemply.
 
Cheers that gives more options and might have some clessetite in the garage. Must say I like the idea of no sealant and perhaps if I could fabricate in the workshop and then fit I might give it a try but as is the back up of sealant seems prudent.
 
Loctite 577 is an excellent thread and pipe sealant, and works well with PTFE tape. A new olive on a compression joint should seal perfectly well dry, but if re-using a couple of wraps of Ptfe around the olive covering the witness mark one from its previous seal will allow it to go again. A light smear of 577 finishes it off, especially if it deeply grooved.

On taper threads 577 is all that is needed.

Note wrapping the tape around the male thread a compression fitting nut does up onto will provide no sealing affect at all, just make the thread tight to do up.

PTFE tape is better cut with scissors than torn as this makes it shred. When wrapping a thread follow the natural direction as though you were following up the thread with your fingernail. This way when you screw it into its mating part the Ptfe won't unwrap before it comes into close contact to perform its sealing task.

Gas-tite is pretty good as a pasty sealant, but I generally use Ptfe with 577 where necessary. My fittings do not leak.
 
superheat6k said:
A new olive on a compression joint should seal perfectly well dry, but if re-using a couple of wraps of Ptfe around the olive covering the witness mark one from its previous seal will allow it to go again.
Click to expand...

?

Surely when an olive is used the first time, it's locked onto the pipe and not available to be used again?

Pete
 
Generally yes, but each time it is tightened it will cut a little deeper into the tube. In a perfect world olives would only be used once. In the real world this isn't practical, so applying a sealant allows re-use of the joint.

Over tightening is also a problem and worsens the seal, so a compound with some flexibility and good gap filling capability is ideal. Loctite 577 partially hardens when it is tightened by anaerobic reaction when the air is removed.

We use it extensively for refrigeration system fittings up to 30 bar - the stuff just doesn't leak ! Even works on weepy o rings as a bedding compound, especially when a new o ring of the correct size just isn't available.
 
superheat6k said:
Generally yes, but each time it is tightened it will cut a little deeper into the tube. In a perfect world olives would only be used once. In the real world this isn't practical, so applying a sealant allows re-use of the joint.
Click to expand...

Ah, I see - you're not talking about using the same olive on a new piece of tube, but undoing and redoing the joint with the same tube? That makes sense.

Pete
 
Hi Trubdlebug. I am looking for a two way switch over valve for my fuel system so that if the filter becomes blocked I can route the fuel through another filter connections are all 8mm copper tube.
Regards Mike
Trundlebug said:
I would endorse what's been said above, NO SEALANT on copper compression fittings, just PTFE on the taper threads.

I replumbed all my fuel lines last year and fitted a filter changeover system, so lots of valves, pipes, and joints.

No leaks.
Click to expand...
 
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