Tank tunnel widening technique

[mattsway]

Hi. I have a CB200 tank to go on my XBR500 build and need to get clearance where my frame rails attach to my top tube. At this location, the tank tunnel is 130mm and if possible needs to become 160mm. Having googled around, I hope to be able to use a plastic mallet. Any advice on weight/shape of mallet and technique - do I strike against the inside face of the tunnel first, or the bottom edge where the wall and bottom of the tank meet? Any prep to help move the metal most effectively? I thought about filling with boiling water, but I could always purge the tank of petrol/gas fumes and use a butane torch. Many thanks.

[CaTacL1sm]

That tank is 2.9 gallons, down from the 5.2 gallons on the XBR500. Are you prepared to wind up with a 1.5 gallon fuel tank?

don't purge fumes with fire... if you must do something, rinse it out several times with water first

[woodsman]

We're going to need to see the bike to know the best way to get it on.

[hillsy]

Good luck with that - I tried to dent the tunnel on a GS650 tank to fit over the head of a 450 and only had to move part of the tunnel less than 1/2 inch each side in just a small area and the outer skin still deformed.

You'd probably be best off cutting it out and re-welding.

But I doubt you are set up to do this seeing you are asking here.

[irk_miller]

If you're referring to the the black circled marks in the pic, all you need is the round side of a ball peen hammer. I assume by plastic mallet you mean a teardrop. A teardrop won't work as well here. I work mostly with a repoussé and a ball peen for work like that.

[mattsway]

If you're referring to the the black circled marks in the pic, all you need is the round side of a ball peen hammer. I assume by plastic mallet you mean a teardrop. A teardrop won't work as well here. I work mostly with a repoussé and a ball peen for work like that.

You're right, it is the areas circled. I suppose I need to reverse the corner. No problems with losing capacity, I can't ride far.
I was thinking to use some oxalic acid I have handy to derust and purge the tank. Any chance you could link me to the hammer you would use for this job, as I don't know what face size, weight is best? Will it need heat? Many thanks.

[TrialsRider]

Rust is the process of iron in the steel turning into soil. Your tank was originally cold formed supposedly using new steel, now it's old rusty steel so it is thinner in places then it was originally, if you just try to hammer it into a different shape I would expect a better then 50% chance you will end up with a leaker. :I think you need a new plan.

[mattsway]

A couple snaps.

[mattsway]

Rust is the process of iron in the steel turning into soil. Your tank was originally cold formed supposedly using new steel, now it's old rusty steel so it is thinner in places then it was originally, if you just try to hammer it into a different shape I would expect a better then 50% chance you will end up with a leaker. :I think you need a new plan.

I got it for a song, so if it fails and I can't get it brazed, it's no big loss and I might learn a bit along the way.

[TrialsRider]

Beauty truly is in the eye of the beholder, because from here other then going from a sticker to a tank badge, the change looks more like a down grade imho.