I fixed a dent yesterday in my tank using a spot welder i purchased from harbor frieght.  It basically welds nail type things onto the metal and you attach the provided pull hammer to it and pull it and get the dent out.  It took me 11 nails to get mine out.  The only complaint is that the hammer wedges a wheel with an off center axis inbetween itself and the nail to hold on and the wheel some times gets wedged and you have to pull the pin out and take it apart and reassemble it in order for it to work correctly after that.  but the dent is gone now and all I need is to bondo it where it is sort of rippled.  

here is a link to the one i bought
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=3223

now all i have to fix is that tail light because my bro backed into the bike and knocked it over.  I think i am going to go and do an led tail light.  I will post pics when I am done.

i have pictures but do not know how to post them in a post, i have tried before but they dont work correctly.  

I was relieved to see this because i thought i was going to you have drill into the tank and pull it out that way.

I wonder if you could just deliberately stick an electrode or two to a tank dent, then grab the electrodes with a pliers in each hand and pull.  I've heard that some dents don't take much outward pressure to pop back out again.

(not that I have any tank dents to test this with)

i have plenty of dents to test with but no welder. some of the dents though actually are creases and i don't know if i would be able to use that kind of method to fix a crease. I really hate the crease and was thinking about just using bondo to fix it but i might just take it to a body shop to be pulled like you said you fixed yours.

Hm, no welder.  Welding tanks is bad anyways.  How about (OK, this is worse) controlled explosion:  you clean-out the tank, close the petcock underneath, plug-up the tank cap vent hole, then you dump in: 1/4 cup baking soda, then 1 & 1/3 cups vinegar mixed with 2/3 cups warm water (should fill a 2 cup measure)... and close the lid, shake the tank, and take off running.

OK, probably not a good idea, unless you figure-out the PSI this would create inside.  That sounds like a job for a chem major...

I wrote this all out and then realized I'm a big nerd. Oh well, maybe you'll find it interesting anyway
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calculating the pressure is straightforward. It's only hard because you've given me units of cups and PSI.

Vinegar = HCOOH. It's acidic, so it gives off H ions, leaving you with COOH-.

Baking Soda = NaHCO3. It's basic, so it gives off OH-, leaving you with Na and CO2. The CO2 is a gas, which is what causes pressure. The OH and H combine to make H20 (water), which leaves you with COOH- and Na+, which bond together as a salt.

So, for each Soda and vinegar molecule, you get one molecule of CO2.

Assume vinegar is 1% acid. That means you'd have 1/300 cup acid, or  .0008 liters. Assuming vinegar has the same density as water, that's  .8 grams of acid (1 cm^3 = 1/1000 liter = 1g with water) Since the weight of HCOOH is 46g/mole, you'd have  .02 moles.  (1 mole is 6.022 X 10^23 molecules)

1/4 cup baking soda is 60CCs. It has a density of 2.15 (according to chemfinder.com), so 60CCs would be 129g. Baking soda weighs  84g/mole, so you'd have 1mol+ baking soda. Since you only have .02 mole of vinegar, the baking soda is in massive excess. (remember, moles tell you how many molecules you have. we only need a 1:1 ratio)

So, .02 moles of vinegar will become .02 moles of C02. 1 mol of gas at sea level takes up 22 liters or so. So .02 moles would be 1 atmosphere in
.044 liters. Since the tank is  19 liters, it's going to be .044/19 atm or  .03PSI

.03PSI is not nearly enough. Assuming you had 10% acid, you'd get .3 PSI. So use very strong vinegar, and use 100x as much as you specified, and it might work.

Alternatively, you could combust something like gasoline. One molecule of octane gets converted into 8 of C02, but you consume 12 oxygens in the process. Luckily, the combustion is very hot, so the gasses get to much higher pressure.

One mole of octane is ~1/3 cup. You'd need 12 moles of oxygen, so you'd better pressureize your tank first. Wait, if you pressurize the tank in the first place, the dent would come out. Never mind.

So it turns out Chemistry was actually useful. Thanks to google for the helpful unit converstions.

Using any kind of internal gaseous pressure to remove dents will not work. The weakest part of the tank will expand first, which are usually the seams.

Quote from: Rema1000I've heard that some dents don't take much outward pressure to pop back out again.

Dry ice will take care of those.

Dry ice may work on small dents but I have never had any success with it on larger dents. One of my tanks has a small shallow dent which  I may try using dry ice again.

Ja, it only works on minor dents, but it's easy (and free, if you get deliveries in dry ice) to try as a first step.

Quote from: Roadstergal....(and free, if you get deliveries in dry ice)....

You suck!    
I have to pay for it. It is fairly cheap from a grocery store though.

WHAT EVER YOU DO, DO NOT USE THE DING KING

Yeah cuz I totally F'ed up my tank, it pulled the friggen decals up of the tank, so now my bike is a SLZUKI


ANY ONE GOT A YELLOW TANK AND FENDER?
(yeah I know a shameless plug)

Bad Aaron, Bad Aaron(whap)

how do you remove a dent with dry ice? oh and does it hurt the paint

Quote from: aliveandkickinhow do you remove a dent with dry ice? oh and does it hurt the paint

Heat the area first and then rub the dry ice on it. The temp change shrinks the metal on one side and the dent should pop out. If the dent is fairly new then the paint should be fine. But if the dent has been there awhile or creased, then the paint may flake off eventually.

use compressed air to fix dented tanks,  remove the petcock and replace with an air nipple,  connect it to an air compressor and turn it on  (works great as long as there is no crease)  also harbour frieght has a store right down the street from me.  My wife says I'm not allowed to go there because I always blow tons-o-cash on crap I dont need.

Quote from: ktrimuse compressed air to fix dented tanks,  remove the petcock and replace with an air nipple,  connect it to an air compressor and turn it on  (works great as long as there is no crease).....

That is a good way to completely ruin your tank. The weakest part of the tank will be affected first. The only way it will work is if the dent is the weakest point, probably not. I have seen seams blow out and others that blew up into a big ball.