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MG MGB Technical - New Fuel Tank

My fuel tank is leaking. Usual thing - smell of petrol when full and seems to leak from where many seem to eventually corrode - along the top. Not complaining as I fitted this one 26 years ago. Mild steel construction and I painted it all over with Hammerite (for what it's worth) before fitting.

Now I have a choice. For exactly the same money I can buy another mild steel one with drain plug and all associated bits (the old one didn't have a plug), or a galvanised one without a drain plug, but still with all the bits. Why I'm still pondering this is that there's galvanising and galvanising and it could still eventually rust. On the other hand, a drain plug could be useful (particularly when changing the tank!), but then the tank is only as protected as what I paint it with.

Anybody any experience of relative merits, or got thoughts on the subject?
Peter Allen

Yes - go for the galvanised one - the issue often is not the water on the outside, but condensation inside that rots the tank.

I fitted them to my fleet (3 cars) over fifteen years ago after painting them on the outside with black hammerite - no signs of corrosion and definitely no leaks.

Never needed to drain them either.
Chris at Octarine Services

If by "galvanised you are referring to the Canadian made "plated" tanks. Both mine have them, BUT they are made from plated steel, they are not a Galvanised (hot zinc dipped) tank. They have one weakness if you are after a replacement for the earlier one with a separate fuel outlet.
The square block outlet is soft soldered in place, rather than brazed, as on the OE items, and is therefore a weaker joint.
You have to be ultra careful doing up the compression joint.
I also cut the solid steel pipe to the pump and inserted a rubber section so that pump movement couldn't stress the joint.
After the first joint failure I haven't had an issue.
Later tanks where the outlet is part of the sender unit don't have this issue of course..
Allan Reeling

I've also read of a problem with Canadian tanks where the filler pipe extends down into the tank an inch or more, which reduces the capacity. Easy to check by comparing the inside length with the visible outside.

FWIW I replaced mine with a standard steel, two coats of Hammerite Smooth on the outside then a thick coating of Waxoil on the top. That was 30 years ago.

The old tank showed no signs of corrosion when peering in the filler, even though there was a little water inside. Corrosion needs oxygen as well as water to occur, which is why fence posts always rot at ground level, and when you dig out the old stump it's solid.

Whether oxygenated fuels increase the risk has been the subject of much discussion in many places, and the usual advice for over-wintering for example is to brim the tank - which is only going to affect the top of the tank if at all. But oxygenated fuels are capable of absorbing 50% more water than non-oxygenated, i.e. reducing the amount of condensation forming under the 'lid'.
paulh4

Thanks, chaps. Food for thought. On the subject of the possibly weak outlet, is it possible to put a late model tank into an earlier car (mine's a '72) if one uses the later type of sender? Would there be any other issues (other than a new pipe to the pump), as I need to replace the sender anyway?
Peter Allen

Not sure on that one Peter. The gauges were different, but whether compatible I wouldn't know. Maybe Paul does.
Allan Reeling

As far as I'm aware the middle and late tanks mount the same, the gauges are different as said, but the late senders are also fit the earlier tanks if you seal the outlet.

On the MGOC forum various people have posted max and min resistance readings of both types, as well as the original metal types and the plastic. A replacement OE (i.e. metal) late sender with output pipe measured 20 to 258 ohms before fitting, and an after-market plastic without outlet pipe measured 17.5 ohms to 250 so pretty close. Subsequently two people posted that their pre-77 plastic sender ranges between 6 and 250 ohms, but all that would so is make the gauge read higher at F, the important thing is that the E reading is still close. Then last year someone received one that ranged from 18 to 185 ohms which is a lot different at the E end and would almost certainly show a significant amount of fuel left when it ran out. As posted elsewhere the MGOC eventually admitted they had had a bad batch, maybe it was one of those.

The earlier gauges are capable of being adjusted for a large range of sender variation, I don't know about the later gauge, but that wouldn't apply in this case.
paulh4

" a problem with Canadian tanks where the filler pipe extends down into the tank an inch or more, which reduces the capacity"

It's just a thought, but maybe that's not a problem but an intentional way of creating the expansion volume required by various regulations these days.

It's a pretty simlplistic approach though, so maybe it is a manufacturing error.

Also, I've no idea whether the regulations are supposed to apply to replacements for pre-existing systems.
J N Gibson

Expansion space in North American tanks was created by an internal chamber, open at the bottom with a bleed hole at the top. When first filling you could brim the tank but the internal chamber barely filled. Then as the internal chamber slowly filled and the levels equalised the level in the main tank dropped to prevent overflow when the fuel expands on a hot day.

Whilst on the face of it extending the filler neck down into the tank would appear to do much the same thing, there must have been some reason why it wasn't acceptable to the authorities, it would have been a lot cheaper!



paulh4

Wow never seen that before Paul. Did all manufactures do something similar?
Allan Reeling

Some means of achieving the same thing yes. How others did it I don't know.
paulh4

Perhaps that explains why it is so difficult to fill the tank in my '72 MGB. After about half full the nozzle shuts off. After a moment I can get another half gallon in. Wait a moment and another half gallon goes in. Continue like this 'til full. A real pita.

Does the filler neck go into that inner tub?

Thanks. Jud
J. K. Chapin

No it goes into the main part of the tank - bottom left in the attached, which also shows the bleed pipe 'B'.



paulh4

See also the comment above about the extended filler neck on some tanks, as attached.

With the American vented tank you can eventually brim it as the trapped air can escape via the vent and the charcoal canister.

If you can keep adding fuel 'after a moment' it may be you have this tank. I'd have thought that the expansion chamber would fill, and the level in the main part reduce, more slowly than that otherwise it would be too easy to defeat.

Incidentally I read recently that with American filler nozzles with the vapour recovery system if you try and overfill the tank the excess is returned to the filling station tank, but the meter continues to tick round :o)



paulh4

Thanks for all your input on this. Still on a ponder as to whether to go mild steel (possibly stronger, drain plug, but more likely to corrode) or galvanised (but in reality plated, and maybe weaker around outlet and no drain plug). Not too bothered about tank capacity. Good point also about gauges and mitigations against rusting.
Peter Allen

If you get the tank with the outlet on the side don't be tempted to seal it against rusting internally. It's all to easy for the sealant to clog the filter on the pickup, then you are snookered.

Fine on the later ones with the pickup in the sender, of course.


paulh4

Thanks everybody for earlier advice. In the end went for a galvanised (plated?) tank with the outlet in the side of the tank. I noted Allan's point about the possible weakness of this outlet and just nipped it up. Of course, inevitable happened when I filled up for the first time after installation, and the blasted thing leaked from the compression fitting. After tightening, in stages, much tighter than I'd like, the joint now appears leak free. Got the rubber pipe connection to the aftermarket fuel pump, as advised. That top offside front of the tank is definitely a rust trap. On old tank it was perforated in a couple of places, despite Hammerite still being good on most of the rest of the top.

Not a difficult job, but for anybody doing it for the first time, the nearest you can get to an empty tank is sensible - I plugged up the outlet with a spare coupling. Also worth buying the installation kit, so that you've got all the captive nuts, if old ones rusted.
Peter Allen

A note about fitting the later combined pick-up/gauge unit to the earlier tank: the float can foul on the original pick-up tube and not move as it should, it may be necessary to tweak the old tube to one side.
Paul Walbran

This thread was discussed between 14/02/2020 and 06/03/2020

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