Hallo!
Big question:
My petrol tank leaks from one of the screws of the bottom :
Do I fix the hole with some lead?
Do I throw it away and replace with a super shiny tank in aluminum ? … and 400? + delivery expenses!
What do you all think abut it ?
Ciao a tutti
Paolo
I don’t recall there being screws on the bottom. Just studs that are used to bolt the tank to the car. If it is leaking there, then I am afraid you will need a new tank.
There are those that can braze a tank by putting exhaust fumes in the tank while you braze. I have seen it done. Empty the tank of course before you braze. But chances are it is because of rust. Not much you can do with rust.
… you right Tony!
it’s stud not screw and it leaks, two of the three studs .
I can braze but safety first therefore new tank is better … and 400? are gone … damn???
Grazie Ciao
Paolo
don’t be such scaredy-cats! I just pulled the tank from my '46 Studebaker M5 pickup which had rust holes. I took it to a good radiator shop and he cooked it in a tank for several days to clean, then brazed the holes up good, pressure tested, and I then painted and re-installed. Working perfectly fine.
I also seal coated a leaking motorcycle tank on my Indian Chief successfully.
On my M100 Elan plastic gas tank, I plastic-welded a leaking seam successfully.
If I were you, i’d pull the Elan tank, have it tank dipped cleaned, and braze the holes/leaks, finish with paint, consider seal coating the inside, then re-install. That way, you keep it original and solve your problems.
Don’t be easily frightened by gas, fumes, whatever. Go for it!!!
Watched a recent version of Wheeler Dealer on TV, they had a car with a leaky petrol tank and found a local specialist who made a patch and soldered it into place then pressure tested the tank, so have a search locally as there are people out there who do that type of repair.
I found a hole in the petrol tank and made a temporary repair using Plastic Padding Leak Fix. Afterwards I bought a second hand tank but the repair is still holding up over 4 years later, so I haven’t fitted it yet.
I asked for advice at the time: http://www.lotuselan.net/forums/lotus-carbs-f40/fuel-tank-replace-repair-t32729.html
When my tank developed a similar leak I had a ?patch? welded over the whole of the bottom of the tank. It stopped the leak for about three years giving me time to get used to the cost of a new one. I can?t remember what precautions the welder took but it wasn?t anything special and he was happy to do it.
I applied Leak Fix as an emergency repair to the underside of a steel tank that was punctured by debris on the M4 (motorway) at 70 MPH, after having the car towed to the nearest garage and sanding the area down. It worked a treat and I never needed to repair it properly. But that was on a flat section of sheet metal; you might not get such a good result around a rusted stud. You could always try it first - Leak Fix is very cheap compared to the alternatives.
It really depends on how corroded the tank bottom is as to whether a fix to the leaking areas is worthwhile or not.
Pull the tank and clean it inside and out and ultrasonic test the bottom thickness. If it has not lost to much metal then determine how best to patch the leaking areas by repairing the cracks with patching and brazing . if the bottom is not repairable as it has corroded to much then say good bye to 400 Euro for a new tank 
cheers
Rohan
Not sure if you have the Elan or the Plus 2. I bought a new alloy tank for my Plus 2 and was initially concerned that it did not have the dimples in the bottom that go over the diff top mount bolts, but on fitting the tank it was well clear without the dimples. I covered the tank securing straps with large heat shrink to protect the tank from the steel straps. Much cheaper if you can get a good repair done though.
search ; tank aluminum Elan S4 ?
Be careful when Sanding NO Sparks. A friend was trying to remove a Tank on a Ford Anglia years ago, suddenly with a few Sparks BOOM 
. He quickly got out from under the Car with his Clothes on fire 
. He had to do a strip in his Dad’s Driveway while his pride and joy blew up and took his Dad’s Garage Roof off 
His Dad was not was not very happy.
Alan
your final comment is very british alan!..in even more noble words: NOT IMPRESSED! sandy
I feel so lucky now.
The rusty, crapped-out Mini I had when I lived in Germany needed shock absorber replaced. Rear upper mounts in the boot; nudged the fuel tank aside to get to one of them. The line came off the tank when I wasn’t looking, and a few minutes later I smelled petrol, which had filled the wells in which the spare tire and battery sat. And guess where the drain-plug was? Yep, under the battery. You’ve never seen anybody disconnect the earth-cable more carefully! But I managed to get the battery out without any sparks. Don’t tell the Germans that I allowed the fuel to drain onto the ground. I wonder if the statute of limitations for environmental crimes has expired.
I discovered a leak in my S3’s tank shortly after purchasing the car (14 yrs old),
took the tank to a local radiator shop who welded in some new metal,
can’t remember the details as this happened in 1982…
still running on the same patched tank.
I had one of the studs crack on my tank last year, took the tank out & discovered corrosion just in time. Fortunately a good friend is a certified welder. Rinsed the tank out many times with water, then left in the sun for several days before welding / patching . Shipping a gas (petrol) tank to the States is almost as much as buying one…
Here are my photos of the crack & the repair:
nice work Phil
I saw that too, apparently they steam cleaned it to remove the fuel fumes.
Robbie
everyone had me scared to death before I attempted to “plastic-weld” my M100 fuel tank while in situ. to combat my fears, I poured a thimble full of gasoline into an open bottle cap and took my very hot welding/soldering iron and dipped it right into the gasoline. nothing happened. the gas started to boil, but no ignition, no explosion, no burning.
there was gas in the tank when I plastic welded the bottom leaking seam with no problems… just rattled nerves!
of course using a torch with open flame would NOT be recommended or attempted while brazing/soldering a metal tank, so i know it’s “apples and oranges”.