Motorcycle 'prism' tank - shrinkage problems

[Maker]

It may have been an idea to put the filler tube in before the tank became an enclosed item, you would then have had access to both sides with a much better chance of correcting any distortion.

It would also have allowed the use of heat sinks.

I only did a couple when younger and always brazed them.

That's what I was thinking, weld the filler in before welding it together, and do it with some big lumps of copper or ally to draw the heat away.

Oh, and just to be a pedant, that's not a prism.

 

[pedrobedro]

Is it shrinking, expanding or distorting ? Metal expands when it heats up, if it was shrinking it would pull the sides in and when it expands it either has to go up or down and it's gone down. Whichever way it goes it's all distortion as it's changed shape from what he wanted. Put the filler neck in before the bottom and use heat sinks or cooling and alternate the weld. It's easy to get a good run going and think I'll just carry on

 

[Brad93]

If he has pulse on his unit you can reduce heat input substantially by using it.

 

[Burdekin]

The only stresses he's put in are at the weld. The metal does expand when hot but shrinks as it cools. It will just distort the weakest area first but this isn't where the stresses are from welding, it's just the result.

 

[mike os]

Can he make a tube out of sheet to sit under the lip on the filler, weld the sheet tube in then weld the filler at the lip/shoulder?

 

[gt6s]

Pressurise the tank with a air line and a tyre gauge that will blow that hollow right out.

 

[outofthefire]

Oh, and just to be a pedant, that's not a prism.

Correct it's a petrol tank

So quenching is not a bad thing in certain circumstances?

I wouldn't quench sheet steel, the heat has already expanded it uniformly, cooling slowly it'll be distorted but evenly. If you quench it it will cool some parts quicker than others and pull it all over the place. Allowed to cool slowly it will also be in an annealed state making it easier to work.

 

[Paul.]

Compressed air quenching sheet steel is completely different to dunking it in water, with air you can hit it straight after welding to limit the spread of the heat into the surrounding sheet, its not so different in principle to using a heatsink which conducts the heat away as you are welding, I do this all the time with the stainless stuff I make.

 

[jordan1]

There are lighter gauge filler necks available may help needing less heat?.

 

[steveo3002]

as seen on all good hot rod tv shows

 

[Brad93]

Lead fill it if you want to do it properly.

 

[KingMT]

I bought a sheet of mystery steel from the scrap yard for my son to do a 4H welding project a few years ago. That crap won't weld at all. I was teaching him to stick weld & it just keep sticking to the plate. I thought it was him at first until I tried also. I still don't know what it is but it cuts like butter.

 

[Ali]

Thanks guys for all the great advice...I'll post follow ups as they materialise