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NPTF thread diesel pump leaking


cheesegas

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Hi all. The mechanical fuel lift pump diaphragm rotted and developed a hole on my Isuzu 3KC1, called Engines Plus and the diaphragm is no longer available as a part...but they could sell me a whole new lift pump at £890 + VAT! 

 

I've fitted a low pressure Facet electric pump and blanked off the mechanical pump. The Facet has 1/8 NPTF female connections, which have a taper and are meant to seal without sealant, without relying on metal-metal contact at the base of the fitting. However, I can't get it to stop leaking, drips about once every 5 min. On each side there's an 1/8 NPTF male to 1/4" BSPT female adaptor, followed by a 1/4" BSPT male to 8mm female compression fitting to get it to connect with the 8mm copper fuel line. Couldn't find an 1/8 NPTF to 8mm compression fitting anywhere! That all doesn't leak, it's just the thread at the pump.

 

Anyway, I've tried a few different brands of 1/8 NPTF adaptor - brass and steel - and they all leak. I don't want to overtighten it, and the threads in the pump all look good. Some sort of sealant maybe? Any ideas welcome, I'm going mad. Thanks!

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Just now, AndrewIC said:

I’m not familiar with NPTF so don’t shout at me, but does it help that 8mm compression is usually close enough to 5/16”

Thanks, yep I've tried searching for an 1/8 NPTF to 5/16" compression too but the nearest I found was an 1/8 NPT male to 5/16" compression - apparently NPT and NPTF are compatible, but will no long seal on the threads and will need sealant. Hoping to keep it all NPTF but it seems that even that leaks!

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When I replaced my sump drain hose, Beta recommended Bondloc B777, which is described as resistant to motor oil, and which worked. Looking at the Bondloc website, B542 might work, “ideal for fine threads” and “resistant to hydraulic fluids, oils, fuel, and coolants”. Someone will probably be along shortly with a better suggestion!

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Loctite hydraulic sealant type 542 will seal this thread but you need to get both male and female thread dry of fuel for best effect. Loctite do a solvent cleaner or use nail varnish remover and allow it to evaporate.

You can get 25ml bottle of 542 on amazon etc

Take great care with the npt taper fittings as you will easily split an aluminium housing.

The full range of npt to tube is available in the hydraulic industry, smaller sizes are used in the medical, chemical and instrumentation.

Edited by adrianh
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A correct taper thread should seal without help 😀but a bit of sealant should help if help is needed.

There is a thicker PTFE tape which is claimed to be suitable (im sure it is, but no first hand experience) plus lots of modern sealants. The old school choice appears to be "Heldite" and I have found that it works well as long as the "gaps" are not huge.

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Just now, cheesegas said:

Great, thanks all, I'll try some PTFE tape! For some reason I thought it broke down with diesel, but a quick google and you guys have proved me wrong. Will keep it off the first few threads as I don't want it ending up in the injection pump...

 

If you can make the forum search facility work then I think there is an old thread about a Beta JD3 with an injection pump full of bits of PTFE tape 😀

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1 minute ago, cheesegas said:

Great, thanks all, I'll try some PTFE tape! For some reason I thought it broke down with diesel, but a quick google and you guys have proved me wrong. Will keep it off the first few threads as I don't want it ending up in the injection pump...

Aren't the filters between the lift pump and the injector pump

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3 minutes ago, ditchcrawler said:

Aren't the filters between the lift pump and the injector pump

Nope - first thing in line is the water seperator, then the filter, then the lift pump. Usually it's just below the injection pump on my engine, feeding it directly. Facet say that the pump should be after the filter too, I don't think they deal well with crap in the fuel.

 

4 minutes ago, dmr said:

If you can make the forum search facility work then I think there is an old thread about a Beta JD3 with an injection pump full of bits of PTFE tape 😀

Ah I'll have a look, thanks, will apply the tape with caution!

 

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PTFE tape is not really suited to diesel.  The diesel eventually creeps past it. If it doesn't  then you have a mechanical metal to metal seal anyway.

Good sealants are Heldite , which seems to be easiest àto find on line, or True Blue, from Toolstation in sensible sized tubes.  Both need to be given time to set before introducing diesel though.

 

Na

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2 minutes ago, BEngo said:

A school day every day.   I guess it might be best to avoid the sparkly stuff lest the metal flakes get where they are not wanted.

 

N

Unless your boat is painted in sparkly metallic paint, obviously you'd then want to match it... 😉

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20 hours ago, cheesegas said:

Great, thanks all, I'll try some PTFE tape! For some reason I thought it broke down with diesel, but a quick google and you guys have proved me wrong. Will keep it off the first few threads as I don't want it ending up in the injection pump...

Yes that's the correct way to wrap the tape. I can see no reason how it can get in the pump if done in this way.

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10 hours ago, BEngo said:

PTFE tape is not really suited to diesel.  The diesel eventually creeps past it. If it doesn't  then you have a mechanical metal to metal seal anyway.

Good sealants are Heldite , which seems to be easiest àto find on line, or True Blue, from Toolstation in sensible sized tubes.  Both need to be given time to set before introducing diesel though.

Thanks, I'll get some True Blue, there's a Toolstation near me at the moment. All the reading I've done suggests that NPTF is gloriously self-sealing, hopefully that'll solve it. Very difficult to clean the female thread on the pump though...

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4 minutes ago, cheesegas said:

Thanks, I'll get some True Blue, there's a Toolstation near me at the moment. All the reading I've done suggests that NPTF is gloriously self-sealing, hopefully that'll solve it. Very difficult to clean the female thread on the pump though...

I did read up after my previous, and I thought NPTF may not be reliably resealable though?

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I find a  (trimmed down  as needed) toothbrush is pretty good for clearing out female threads.   Or, better, put a plug tap of the correct size down, but I suspect NPTF taps are not in every toolbox.🙂

 

As for resealability, that is why some sealant is a good idea.

 

N

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