Hello, and what is the score with leaking 'wing nut' fuel tank caps?

[Ratty]

Hi all, and thanks for letting me join your most excellent looking forum! I fully intend to get involved and offer as much to the forum as I may take from it, so huge apologies for being on the latter end of this from the start, but;

How do you get a good seal on a 'wing-nut' type fuel filler cap? I have a breathed on M100 Panther, and I love it to bits, but the filler cap leaks like a !"£%^% with anything more than half a tank. Partly I can put this downtown vibration, but they can't have been this bad when new, so what have you all done to get a sealable filler?

I've made sure the hinge is angled right, so the cap fits flat to the filler neck, but it still loosens with vibration. Do I need a better rubber gasket? Or is this just an incurable problem that will only be resolved with a different tank cap?

All input gratefully received 😉

[R]

When you say 'loosen off' with vibration, does your inner workings of the cap have the spring action to it,
such that when you snug down the wing nut/bar, it feels like it sort-of locks down, and then shouldn't move.

Also, my cap has more of a fibre type ring seal to it.
I've seen it said these should be boiled up before use, the moisture then makes them fuel-proof.

But maybe we haven't experienced panther type vibrations, so who knows !!?

Welcome aboard.
Cheers.

[john.k]

i think these caps originally had a cork seal,which is softer and more resilient that rubber......the cork is fuelproof and easily conforms to the gap without force being needed.

[rgsjohn]

Not sure what the answer to this is. On my RGS the tank cap has always been incontinent. I have tried various different materials to seal it. It originally had a rubber seal. So far I have found nothing which works very well. I have just learned to live with it. I just try to avoid sharp braking and bumps when the tank is brim full.

[mini-me]

If its the kind I think it is, with 3 little gizmos expanding inside the filler neck, you will find that the neck is stretched over the years, I have the same problem with pie crust fillers on AMC.

Make a gasket out of soft leather if poss or make up some kind of baffle.

I did once have an idea for an insert and threaded filler, but it was beyond my skills to finish it. Now I never fill past the critical mark.

[Ratty]

Thanks  all for your replies, and apologies for the delay in replying.

Yes, it's the type that has the 3 'wedges' that expand into the neck of the tank (it's a new reproduction tank by the way, so shouldn't be worn, but may be oversized I guess). It seems like the vibration from the motor allows the wedges to slip, and the cap works it's way up from the flat fit it has when locked down, obviously angling towards the rear hinge.

As an aside, the tank filler neck is completely smooth from top to bottom. I wondered if on an original tank there would have been a groove for the wedges to sit in, or a ring for them to sit below? The seal I have is definitely of a slim rubber type, so I may try cork or fibre, to see if this will fill the gap.

Thanks again guys...

[mini-me]

Nothing inside the neck at all, you could try a bit of leather as an alternative seal perhaps?

[john.k]

My M100 from 1946 does have a shallow groove formed into the neck,right where it joins a flanged hole into the tank shell..........and it does grip tightly and pull down as the wingnut is turned,

[Ratty]

My M100 from 1946 does have a shallow groove formed into the neck,right where it joins a flanged hole into the tank shell..........and it does grip tightly and pull down as the wingnut is turned,

Thanks John. Hmmm, wonder if a little gentle metalwork would give the wedges some purchase...

[R]

Norton tanks are just plain in that region.
Mind you, they did go to a twist tap cap in the mid 50s, so who knows what the service dept knew....

Does your cap have a spring action to it when its out loose, and you press down on the inner gubbins.
Its the loosening off thats the problem ?

[john.k]

The Nortons I have (plungers and rigids) all have a very noticable bulge ,quite clearly seen from the outside ,where the neck joins the tank shell.......However the Norton cap doesnt lock 100% securely ,and can be lifted slightly from the neck,but wont come open......I suspect when the tank was new,the cap did lock down tight.....As someone mentioned ,I would never fill the tank to the top,unless there was some compelling reason...........Even the screwdown caps fitted to some twostrokes wernt 100% fueltight ,as leakage from the centre breather hole would occur.In fact I think the BSA bayonet cap type,which is the standard Ceandess cap ,fitted to many things.............In fact I seem to remember the Perkins oil filler cap and BSA tank caps are interchangeable.

[iansoady]

My 1952 Norton ES2 (original tank and cap I'm sure) doesn't have a bulge but the cap does tighten down firmly and doesn't leak. It does have a fairly thick "rubber"  (no idea what it actually is) gasket which covers the whole of the underside of the cap with a hole for the breather.