Needs some tips on fixing this dent

First off, the tank has not had a drop of fuel in it for over 30 years and has been open for several months, there are no fumes inside thank god. As you can see from the pictures it's a pretty bad dent ontop. I do not have a stud welder, just my MIG. I am pretty darn good with it, and was wondering what would be the best way to go about removing this dent. My biggest concern is burning through the metal, of course. I've welded thinner materials with success, but i've always been able to use a heat sink. I am thinking if I flip the tank on the ground and fill it with water half way that would act as an excellent heat sink and I could weld on several studs. Option 2 would be to place a flat steel plate over the filler neck opening and clamp it down as tight as possible to form an air tight seal, then plug up any other ports and start pushing some air into the tank. Though I don't want to bulge it in the wrong places. Would it be possible to cut out the affected area, hammer back to shape, cut to fit and weld the section back in? With enough patience I can usually turn out some water tight welds. The tank has 3 baffles inside, so i'm limited on arm room.

Most body shops have spot welding set-ups that attach a stud which can then be pulled to straighten out dents and dimples.
How do you feel about welding up small holes in the tank? You can make a "plate" out of 3/8" steel or even a 2 X 6 long enough to bridge from the end of the tank to about it's midway point.
Lay it across the dented area and drill through the plate over the deepest points of the dent(s). Drilled holes should be big enough so that #10 self-tapping sheet metal screws will spin in these holes. Next run the self-tapping sheet metal screws through the holes and into the tank dent. As you tighten the screws, they should pull the dent back out.
Use several #10 or bigger ascrews and grease the underside of their heads so they don't bind. Alternate between them to keep the pulling load even and spread out. Some 1/4" spacers under the plate will let you over pull the area so that it'll spring back to level when you remove the screws.
When you're satisfied with the results all you'll have to fill/plug weld are the screw holes.

Jones, great idea! I have some thick steel plate laying around that I think would do the job nicely. Instead of filling in the screw holes I could just thread the screws back into them just a few turns, cut the ends flush and spot weld them in, I think this would work better than trying to fill the holes without the ability to have a plate of aluminum underneath them which is how I normally fill large gaps and old screw holes. It's actually similar to an old gunsmithing trick I have used to fill holes in tapped receivers and barrels. Locktite in a threaded screw into the hole, cut off the excess, peen the head, and file flush.

I have popped dents out of tanks before using air pressure. It won't take out a crease though and you have to be really careful to not blow the tank up. Fuel tanks won't take much pressure (less than 10psi).

Personally, I believe I'd just try and find another tank.

Yes, to what Jones suggested!
Did I read "... pretty darn good" with a MIG welder...? Welding up a few small holes should be no biggie than...
Do some practice welds first, on a piece of #18 or better #20 gauge sheet metal. Remove rust or paint around holes before welding!

Another alternative would be pressurizing the whole tank after building a strong crate, to firmly support the heads (plus all flat surfaces), but using WATER, not air....!!!
Safety First!


G.

You might consider welding a thin metal plate to the tanks radius edge, then pull the plate with a come along while working out the tension with a body hammer. You can do that with your mig, I use similar with Mo-clamp plate weld puller. Nothing will replace the knowledge gained from experience, I've done this over 20 years.

AMX, I know you have repaired a "couple" dents before, your technique sounds interesting. Could you be a little more specific? I'd rather not pick up a new tank, they are difficult to find for the GMC's and aside from a little damage there's nothing wrong with this one. I'd rather save the good parts out there for people who are doing serious restorations. Mine will never be original again so I make due with what I have. I'm even doing some cut and weld work on the tailgate to repair some smashed areas.

The (radius edge) is the curved edge that runs length wise of the tank (and truck). you can cut a piece of light steel plate/sheet to fit the curvature of that lengthwise dent and weld it to the tank. A pair or two of vicegrips clamped to the plate will give you something to connect a come along to (you dont need a very strong connection, because pulling too hard will rip the tank). Pull tension on the plate slightly while tapping on the crease lines of the dent (on the top and sides). As you relieve tension the pull on the come along can be increased. As the edge of the radius reaches the correct height you will need to cut each end of the plate (alittle at a time), so that your pulling force is being used only on portions of the dent that are still low. After you have re-established the radius edge, you may have to raise smaller dents between the radius and crease lines. you can use narrow sheetmetal strips with the vice grips in a manner similar to stud welders.. Just do not try to inflate this tank to pop the dents out, iot will not work. Ps, If I had used proper terminology I would be calling the radius edge a "crown". Hope you got something out of this.

By the way there is no problem with using a little body filler on properly prepared metal, once you have gotten the tank crown re established. Preperation is the key.

If its not going to be a show truck I would leave it alone and just say it adds character. I'm thinking about trying to
take a few small dents out of an old motorcycle tank and I was thinking of similar ways to do it, but if I screw it up the
cost of finding another tank would probably more than the bikes worth.

My tank on m211 looked good and after cleaning it out best I could and refinishing it when I started using the truck it started leaking through pin holes that keeped popping up . I would use it and repair it later .

Understood every word, excellent instructions! I will make a cardboard template of the crown tomorrow to start with. I am fairly new to MV body work but in the 1 year I have been working on the truck I have learned quite a lot. The floor was the biggest challenge for me. Years ago the truck was picked up by a forklift when it had no axles and was a bare cab/chassis truck. The fork made a very large and high dent in the middle of the cab floor.
I know metal stretches when bent, and one cannot simply bend a piece of steel back and expect it to lay flat since there's now "extra material". The first thing I did was cut the beam that runs under the cab floor. This is where the fork made it's damage and did the pushing. With the beam cut, I pulled the floor back down using C clamps and a section of square tubing laid across the underside of the truck. Course I knew once released it would spring back up a little, so I pulled it further down than necessary. Before releasing the clamps I then welded the beam under the cab back together.

It worked great and the floor is darn near perfectly level again. There's still a section of sheet metal on the cab floor that is bubbled up and seriously distorted in several spots. I found the easiest approach to fixing this will be cutting it out and just welding in a new piece. I've gotten to where I really enjoy doing body work and I would appreciate any tips you could give me later on the subject. Pretty much a jack of all trades, patience is the biggest reason why most people can't do stuff like this. That, and you must have a can do attitude.

Glad to hear that body filler is not taboo, after repairing a few cracks in the hood I went back and filled in any imperfections with silver solder and a hand file. Much more labor intensive than body filler which is very forgiving to work with. Do you have any recommendations on a good quality filler that can be easily obtained?

On a side note, I am aware that a MV cannot be perfect. I am fine with runs and drips here and there, but I have been busting my rear end going over the entire truck to make it really, really nice when finished. Always been a stickler for a quality job. With a final coat of 24087 OD it is going to look like it rolled off the GM assembly line.

Like motorcycle fuel tanks
nice to have ideas - get yourself a 1/2 bar stock and remove both the screwed on items. fill the tank 1/2 with water.

Use the bar through the access port to use like you are going to break into a locked up vehicle. One end will have a curved end on it that is peened over on the tip as to not pierce through the tank.

Lightly heat up the dented area at the radius first and start prying up on that section once little area at a time. That will nearly tank the dents from the flat panels - then just go back and work on those.

If there is anything small left in it distorted - use Duraglass which was commonly known as tiger hair yrs ago - much sronger than regular ole bondo - Bondo has a product like it as well.

I have used a basket ball to pop a dent out of a car door! Deflate insert and re inflate till dent pops! Just and idea!

Any of those tanks have a sealer material on the inside, to prevent corrison. Any welding will burn that finish, leading to future problems. Welding will also cause distortion around the immediate areas. Id stay away from welding. If you want draw studs, solder them on. Clean surface till its bright, and solder them where you want them. File the head of a 1/4" cap screw, dead flat, tin it, then solder it on. Use a slide hammer to pull the dent. Also use a slow steady pull. Also use air pressure to "Help" pull the dent. You will only use a few pounds of air to do the job. You start welding on that tank, you might as well, throw it away now.

Negative on the sealer, it has been bone dry for 30+ years and it has a fair amount of rust inside from sitting. Mostly surface rust and what I could get with a power washer it took it down to bare metal quite easily. I'll be using some Evapo on it later.

I like the basket ball idea, however this dent is behind both inner baffles. If it was a shorter tank and my arm was not as big as it was, I think the other method of heating the metal up would work too. I'm going to make a cardboard template of the crown as AMX suggested and will weld on a section of metal to pull on. Suppose I could just place it under the front bumper to get a winch point and fill the SOB with water to weight it down. Weighs about 450 full.

having to ask how to do it - indicates that you probably dont have the skills required to salvage that tank.....the pro would cut out the dented section with either a plasma or abrasive cut off wheels and weld in new steel - formed to match the tank...probably tig weld it.....with the small tig welds they could be properly blended to be invisible

how about taking the tank to a good weld shop and see how much $$$ they want - compare to replacement and make a decision

I am no expert, never taken a welding class in my life. But asking for tips on how to do something in no way indicates a person does not have the skill or knowledge required to perform said repair. I've got a lot of work to get done here and it's been 110+ these past few days, so it will be awhile before I'm able to get back to the tank. I'll be sure to post some pics when it's done.

"I am no expert, never taken a welding class in my life. But asking for tips on how to do something in no way indicates a person does not have the skill or knowledge required to perform said repair."

My apologies sir - certainly didnt intend any negative Karma .......I still think a repair such as this - with the dangers of welding and cutting a closed container that held a flammable liquid and properly welding it back up - with no welding training and having to ASK - rather than feeling confident in his skills and DOING the job and posting finished job pics - speaks volumes....

there are accepted procedures for welding containers that held and will hold flammable liquids and no-one should do this work without being familiar with those procedures....trying to save a few bucks can cost you your life - or alter it forever should you find yourself doing this repair under the wrong conditions.

No offense taken, I always try and keep in mind that on an internet forum it can sometimes be difficult to determine somebody's exact tone. I am very confident in my welding skills, done a lot of fabricating since owning the truck and I've been complimented on my work by those who have been welding far longer than I. That is always nice.