Steel fuel tank dent removal

[David Lahey]

I've got a couple of steel twinshock fuel tanks that have suffered serious denting and am going to have a go at straightening them out so they can be used. The idea is to use low pressure compressed air and a special jig to hold the tank in shape while I use localised heating to soften the steel, letting the air pressure push the dents out. I have borrowed a TY175 tank-holding jig from a friend and seen the results he has achieved on other TY175 tanks using this technique, so it should work OK.

Special safety note here to point out that the tank in the photo has already been meticulously cleaned out to remove every trace of fuel residue before I start heating it. To clean it out, I used clean petrol, then turps, then water, then dishwashing liquid mixed with water, then water again. This tank has no internal rust, but if it had been rusty inside, I would have removed the rust too, probably with molasses.

Here are some "before" photos of the ex-Hagsta TY175 tank. Note the gearshift pattern guide written on the top surface - cool hey?

[TriCub]

Those tanks look like perfect candidates for an open up, panelbeat and weld back together. That's the way I did a repair job on our TL125 tank. Just grind the seem all the way around and the tank comes apart. Then belt out the dents and just weld it back together with a small OXY flame . Bit of bog and paint and Bob's your uncle.

[manxman]

Coming as I do from a land far away, I was wondering just how the molasses cure for rust in tanks works. I have had reasonable results in the past with vinegar mixed with coke (the drinking kind) but have not heard of this one. Do you think Tate & Lyles syrup would be as good on the TL125 tank I am currently soaking???

[David Lahey]

There are probably lots of things that will dissolve the rust in bike fuel tanks but where I live, molasses is pretty cheap at the local produce store and works well albeit slowly. It dissolves the rust because it is acidic.
I work at a large power station and we periodically clean the internals of the (steel) boiler tubes with warm citric acid to remove iron oxide and copper deposits and it works a treat too.
"Rust converter" (phosphoric acid) as sold in paint shops does work much faster than molasses but doesn't have that same lovely aroma as molasses.

[keychange]

David, some questions on molasses

do you use straight or diluted molasses?
how long does it take to treat surface rust inside tank?
once treated should the tank then be sealed to stop more - if so with what ?

Any other tips

Thanks

Andrew Hunter

[David Lahey]

I find it works best if the molasses is diluted just enough to make it flow freely. Some Montesa fork tubes that were rusty inside took a couple of weeks to dissolve the rust and from memory my KT fuel tank took about one week. Both were probably done during summer and the process seems to be temperature dependent.
No coatings needed after the molasses. Just rinse all the molasses off with water and spray with dewatering liquid to get the water off the steel. Blow as much as you can of the water droplets and dewatering fluid out with compressed air and leave to dry somewhere warm. If it is a petrol tank rinse it out with petrol before using it.
I would not recommend using any coatings inside a steel fuel tank - a rigid coating will be broken free of the steel if you dent the tank, and flexible liners can come loose after a while and are hell to get out through the filler hole.
I did my KT250 tank about eight years ago with molasses and it is still looking good inside. I think if you keep the amount of water inside the tank to a minimum the corrosion rate will be very low.