Portable plastic fuel tank gauge bung problem.

[CliveF]

Ok so its a 22lt Nuovarade "hulk" Polyprop tank, In a T32 Honwave
4 years old never been a problem until i went to fill up this time.
The clear window over the gauge was in place but not attached.
As i picked it up after filling to put in the car petrol flooded everywhere.
Back at the temporary residence I managed to find enough things to investigate. ID of the thread in the tank was 34mm ish ( some irregularity )
The OD of the bung was only 34.14 ish so virtually NO thread engagement.
I tried PTFE tape , which failed, I suspect i did not put enough thickness on.
Next I managed to get a fairy good engagement on put a ratchet strap round it and drove some slips of wood under to jam it in place. that lasted for the two short trips i did.
Now back home I want to make a Proper repair but what with.
I am inclined towards the RTV 2 stroke crank case gasket sealer I have which is setting type but not sure if OK for constant fuel contact ( fuel does slosh up into the clear bung.)
Or build up a much thicker layer of PTFE tape to see if that will give a better grip.

Hylomar blue I have , it is fuel proof but non setting so no mechanical grip to hold in place.
Loctite 577 also fuel proof but £15 for 50ml is nearly a new fuel tank ( that may have the same problem.
Permatex also non setting so no mechanical grip.
Epoxy glue is not fully fuel proof.
Gorilla glue is NOT recommended for fuel contact

Does anyone have have any reliable suggestions that THEY have actually used for how I might fix this problem, that I have yet to consider.
Nothing I would trust has appeared on YT.

Thanks in advance
CRF

 

[andsarkit]

I feel for you as I once had a diesel can leak in the car and it took a real effort to clear the smell from the carpets.
How about a Jubilee clip around the cap and put it somewhere warm for a while in the hope that the plastic will deform sufficiently to grip on the thread.
The problem might have been caused by the recent hot weather allowiing the plastic to soften slightly and deform outwards.

 

[Boathook]

I had a tank a bit like that years ago. Never could seal the gauge bit successfully and ended up binning the tank.
I just think that the tank was to cheap and money saved on the build quality.
It was a shame as the tank slotted into a space I had.

 

[CliveF]

I feel for you as I once had a diesel can leak in the car and it took a real effort to clear the smell from the carpets.
How about a Jubilee clip around the cap and put it somewhere warm for a while in the hope that the plastic will deform sufficiently to grip on the thread.
The problem might have been caused by the recent hot weather allowiing the plastic to soften slightly and deform outwards.

nice try but the bung is small compared to the thread moulded in the recess of the tank.
I need to expand the diameter of the bung threads.

thanks for the answer

 

[rogerthebodger]

Post a picture then we can see the issues

 

[CliveF]

Post a picture then we can see the issues

not possible as the clear threaded bung sits into the thread moulded in the tank, as I said in post 4.

all you would see is the clear bung resting flush with the top in the orange fuel tank.
You would not be able to see that it has no thread engagement ( post 1 ) and pops out very easily.

 

[ducked]

More PTFE is probably your best bet, though other solid sheet materials (thin polythene, clingfilm, aluminium foil, nitrile rubber from disposable gloves, for examples) might have potential and could be tested by/for long term immersion in a clear glass screw capped bottle. Famous Grouse "hip"stylee with the label removed, for example. Any "sticky" tape runs the risk of putting the sticky stuff into your petrol

I dont know of any goop that stands up to modern petrol long term, though I have fixed petrol tanks on cars apparently successfully long term with both polyester and I think epoxy based fibreglass in the distant past.

A rub with a bar of soap used to be touted as a petrol sealer but I dunno what its current status is

 

[Boathook]

More PTFE is probably your best bet, though other solid sheet materials (thin polythene, clingfilm, aluminium foil, nitrile rubber from disposable gloves, for examples) might have potential and could be tested by/for long term immersion in a clear glass screw capped bottle. Famous Grouse "hip"stylee with the label removed, for example. Any "sticky" tape runs the risk of putting the sticky stuff into your petrol

I dont know of any goop that stands up to modern petrol long term, though I have fixed petrol tanks on cars apparently successfully long term with both polyester and I think epoxy based fibreglass in the distant past.

A rub with a bar of soap used to be touted as a petrol sealer but I dunno what its current status is

I tried PTFE on my tank but the thread fit was too bad to make any difference, the same problem as the OP's.

 

[ducked]

On a motorcycle brake fluid reservoir where the rubber surface confirming bellows gasket thing was swollen past usability (possibly incompatible silicone-based brake fluid had been used) I improvised using I think clingfilm (thin polythene dust sheet plastic, or just suitable guage polythene bag material, might be better/more inert alternatives, particularly for petrol.)

Once I'd trimmed it roughly with scissors, I melted the excess off with a biggish lighter. It formed a bead along the edges of the fluid reservoir, possibly helping seal it and anyway looking tidier.

In your application as you describe it, with no projecting flange on the bung to get in the way, you would/could have an annular wrap of thermoplastic sheet material projecting vertically out of the gap as a cylinder, which you could them melt down into the gap to help seal it.

PTFE is too heat stable for this use and toxic when charred, but you could add a bit of PTFE later

You would of course have to consider the possibility of igniting petrol vapour and explosions, and what you think you can get away with in terms of tank flushing, filling with water or inert gases, or perhaps substituting a non-flame heat source such as a soldering iron.

 

[ducked]

And/Or

You could perhaps psuedo-helicoil it, with either wire or nylon monofilament fishing line.

If feeling brave you could heat wire (which I suppose could be either copper or stainless) either by electrical resistance heating, running current through it, (which likely means just on the bung, since it.d be difficult to get electricity to both ends of the wire in situ with the bung installed). or by running a soldering iron along the wire coil while pressing it against the surface of either the bung or the tank neck.

I'm assuming here the tank and/or bung are thermoplastic.

Triangular or square section wire might be better but I doubt that is available