Tank tunnel widening technique

7083 Views 23 Replies 11 Participants Last post by mattsway, 7 mo ago

mattsway

Discussion Starter · Dec 18, 2018

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.

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mattsway

Discussion Starter · #6 · Dec 18, 2018

irk_miller said:
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.

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mattsway

Discussion Starter · #8 · Dec 18, 2018

A couple snaps.

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mattsway

Discussion Starter · #9 · Dec 18, 2018

TrialsRider said:
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.

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mattsway

Discussion Starter · #12 · Dec 18, 2018

Thanks for the tips on striking. Yes, beauty is in eye of the beholder. The boomerang comstars are going as well. If I can't get the CB200 tank to fit, I've got a CB350 which would need less work.

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mattsway

Discussion Starter · #13 · Dec 18, 2018

Cb350 tank, but not as attractive to my eye...

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mattsway

Discussion Starter · #15 · Dec 19, 2018

seanbarney41 said:
Can I get those boomies?

I'm UK based?

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mattsway

Discussion Starter · #16 · Jan 2, 2019

irk_miller said:
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.

Your advice worked well, thank you. 30mins of fast, short strikes produced the results in the pic, along with a resolve never to engage in an arm wrestle with anyone who shapes metal this way on a regular basis!

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mattsway

Discussion Starter · #17 · Feb 2, 2019 (Edited)

I went to the local shop where the petcock bung was TIG welded into it's new position. The guy says the tank is old and thin and has to switch to MIG to build up a weld over the two original petcock holes. On testing, the weld is leaking at it's base.
My question is whether a tank sealant like POR15 will sort this out, or has the metal on this this tank become too porous and so beyond help? Thanks.