How do you widen a tank tunnel?

[jimson]

G,day viewers how would you go about expanding or widening the the underneath of a petrol tank? Jimson

[Davey Crocket]

Whats it made of?.

[paco]

Cut and shut.......Use the bottom of another tank.........Copy the whole thing in fiberglass.p

[bazza]

allo-steel-f/glass/plastic?

[firko]

It's a big job, no matter what the tank's made of Garry. As Paco says, one way is to remove the whole bottom and cut and shut one with a wider tunnel from another model. That involves a lot of precise welding to prevent the tank from turning into a distorted mess. I'd tack the bottom in and then stitch weld it in alternate positions to try and prevent distortion. You'd better be a good welder too as the last thing you want is a leaky tank over a hot motor. Another, possibly simpler way is to cut the tunnel itself out and fabricate a new one from 18g sheet or cut one from a suitable donor tank. You won't have as much of a distortion problem this way but it may be a more difficult fabrication and welding job. I'm presuming this is for your CB Honda bobber....what tank are you trying to fit? GMC, Yamaico and other metal craftsmen might have another suggestion but I'm sure they'll advise that it's a pro job, not for the back yarder .

[jimson]

Sorry tank is made of steel. Yes Firko its for Hannibal. I was trying hard to do most the work myself but my skill level is failing me. Jimson

[Slakewell]

How much does it need to move to make it fit? We might be over thinking it.

[Mick D]

Thats the go.

Is it the case that it is just not fitting over the wide ignition coils Jimson?

As Slackwell has suggested, if you don't have the gear, just beat it and bog it. No one will see it underneath.
If there is a fair bit of manipulation to do, and you are worried about kinking it out of shape?
Just fill it completely full of sand first before you give it the ball peen.

It should be a walk in the park after that neat job you did of your ute's three bike rack.

Good to see you got home keen and aren't waiting till the week before the first race.

[firko]

What tank are you trying to fit Garry? If it's a close fit the boys might be right with their boilermaker big hammer method.

[Mick D]

And you wont end up owing anyone a  favour

[matcho mick]

carefull how you go jimson,big fugging kabooms taken out a few "home handymen" playing with tanks!!
i was welding up a leaking kombi tank,when the car i was using to purge the tank with CO2 ran out of gas,fugging kaboom bigtime,

[Mick D]

What did you do next?
  Throw your undies away?

[jimson]

Yes I thought about using a hammer. But then I thought about pumping air into it until it expanded a couple of mm then having a brake and starting again.  Jimson

[oldfart]

keep air away from your tank .....it will end in tears 
As suggested cut the tunnel out on the inside of tank using a dremal with  mini blade's as this will leave the seam in place ( holds tank in shape )
Your front triple mount section ( U )  will dominate how much you can spread the tunnel  10 to 15 mm is about max before it starts deforming at the top, in most cases you only need to apply SFA heat at the rear ( frame intersection point )  to open it up.
Either use the tunnel you cut out and reshape it, or roll up a new one.     

[jimson]

Thanks for or the advice everyone.  I think I,ve found a solution to my problem. Just a bit of smoke & mirrors and a sheet of alloy might let me get away with my choice in tanks  Jimson