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The business I used here in the USA for fuel tank repair, always gas welds them as the factory did. He says it produces much less distortion than electrical welding. He uses a very small acetylene torch like for jewelry making.
He is retired now but had been welding fuel tanks since the 1960's.
FWIW I use a Dillon ( now called Henrob 2000 ) with the No 1 or the No.0.5 tips 1 = 0.030" thickness to .250" 0.5 tip is < .030"& I have welded beer cans with it ( badly with a few holes ) to win a bet .
Tiny thin flame that creates little distortion I can weld things like mudguards without distortion
The welded tanks were rolling resistance welded , think of a spot welder with wheels in place of pointed tips .
The big problem with electric welding is not the heat produced during the weld , it is the lack of heat and the speed of heating & cooling.
This can leave the weld stressed and in use becomes quite liable to crack from the vibrations.
Gas welding, while it heats a bigger region heats it slower so you get a much wider Heat Affected Zone but there is less distortion of the crystal structure .
If you are going to join some metal then that piece is going to be formed, you always gas weld as it leaves the entire welded region softer than electric welding.
If you want to stiffen alloy plae a common method is to do some MIG / Tig welds in the direction that you do not want it to bend .
By modern standrds , BSA tanks are made of rail way line .
I'm with Trevor on this one, done correctly gas welding should give a much more malleable weld.
A nice little test of you welding technique is to butt weld a couple of small pieces of sheet. Say 150mm x 50mm.
When cooled beat a cup into it over a ring with a ball pein hammer. You should be able to form quite a deep cup without the weld splitting.
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For years I've been friends with Terry and Drew Roberts who lots will know as "Metal Magic" Amongst lots of other things, they make complete petrol tanks for any make including Brough. They use a MIG for tacking and then everything is gas welded.....I just love watching them.
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I've had 'tinbasher Bob' repair all my sheet metal work for decades..He's based in Birmingham and is an 'old school' sheet metal worker. He repaired much of the tinware off the bikes that were damaged in the NMM fire....He's semi retired now and for some time I've been trying to improve my own skills...Bob gave me some useful instruction and helpful tips, he even donated a 'slapper' to the cause and taught me how to drive it....
He manages most petrol tank dent repairs without cutting, working through the filler cap hole with an assortment of 'balls on sticks' and a slapper...
As always, it's fascinating to watch someone with such good manual skills...He'll remove 3 or 4 dents from a tank in about 15 minutes and chat to you while he's doing it!....Ian
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