Most of us have experienced fuel coming back out through the filler cap while driving, so what is a good solution to sealing a fuel tank?
Is it a matter of getting a better quality cap or can you simply get some fuel friendly rubber and add rings on the cap until it stops

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Respect mine and I'll respect yours.

The only one I,ve heard of was a short tube which fit inside the neck with a round/ flat cap on top which the fuel cap was screwed on to , an O ring was put around the tube flush with the flat cap, the O ring was wide enough to sit on the filler neck edge between the neck and the fuel cap.

basically a fuel cap extension, from what I remember , a template was taken of the filler neck, and the plate/cap was made from that , the plate/cap and short tube were stainless steel and TIG welded so the weld was small enough not to interfere with the O ring, from what I recall, once the fuel cap is screwed onto the plate it is tighter to put on compressing the O ring onto the filler neck edge to form the seal, the short tube inside the neck forms a baffle, the idea, apparently, was that you just have to replace the O ring to maintain the tight seal.

The bit I,m not sure about is, was the plate/ cap cut out so it was around the short tube not over it so the weld was on the outside thus making the O ring fit completely flush, it makes more sense.

I hav'nt made one, I don't have a TIG welder.


If anyone makes one , post pictures, and if it looks the goods I,ll buy one just to see how well it works



.

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New caps on unmolested tanks are usually a pretty good seal.
The problems come from people fitting a locking cap, then villains butcher the cap (and the neck bayonet) to get it off and siphon fuel.
My ex-GT RH tank was butchered so bad I cut the top of the neck off. I then bought a fake RH filler neck, cut it to match and oxy welded it on. The weld seam is inside the rubber grommet, so can't be seen.
Note: The tank was drained, flushed 3x with hot water, then left 99% full of water whilst I welded it.
All good now, no leaky.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

I make quite a few new seals for various vintage and classic oil caps petrol caps etc. and always use ACL cork/rubber gasket making material which is 2.5 mm thick. This is a modern day cork/neoprene type material which i have a lot of success with, also good for making sump gaskets etc etc for vintage cars. I have made new seals for my mini petrol caps and they all seal perfectly. But as the doc said you must have a close look at the filler neck, i find you can dress them flat with a small hammer to improve the sealing quality.

Hope this helps.

PS part number on my last packet of material was ACL MR3124
but this is a fairly large sheet and probably available in smaller sizes.

I think I worked out why they always leak, even with a seemingly good seal on the lid.

Put a reasonably small amount of petrol inside a plastic bottle and give it a good shake, then take the cap off. The bottle will have expanded as the petrol vaporised off from sloshing around. This happens for all kinds of liquids, not just volatile types.
I think the same happens in the limited confines of a full fuel tank as you throw the car around. Fuel sloshes up against the fuel filler as you snappily drive around roundabouts to be pushed out by the pressure behind it. The bleed nipple at the top of the tank is too small to allow all the pressure to be bled off very quickly enough, and its just that the fuel is against the cap at the same moment the most gas is being produced in the tank.

Not an easy one to fix, perhaps a larger breather nipple and hose fitted with a catch can (the original is only about 1/8th ID) or like they do in some instances to prevent gassing off of fluids, a floating baffle on the surface of the fuel to slow down the rate of sloshing (not really practical to make in a tapering fuel tank) .......or just keep adding extra rubber to the cap til it stops...or not put in as much fuel... Both the earlier mods are a little difficult to accomplish, but imagine the amount of fuel which must vapourise off through a year from these losses.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

It's probably not much help to you but our 1100 based cars have the same problem, over time we worked out that if we put a certain amount of petrol (20L) in when the guage was at 1/4 full we didn't have any spillage (this also works quite well with the one shot fuel additive we use as it's in measured 20L doses)

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'62 Morris 850 (Marion)
'65 Mini Delux - sleeper 1275/S discs (Ms Plum)
'69 Vanden Plas Princess 1300
'71 Wolseley 1300
'74 Austin 1300 Countryman
'94 XJ6 Sport 3.2
'04 Rover 75 CDTi