Best fix for gas tank dent

[roadahead]

Hey all,

I really need help

I took a pretty good spill on my 78 cb750F last week. Luckily the worst damage was a pretty good dent in the tank. I've read so many warnings against various tactics to fix gas tank dents that my head is spinning. So I figured I would post pictures of the actual dent in question and see what guidance I can receive from this amazing team of people.

I figure the dent is too big to just bondo over. Even if it wasn't I'm finding so many people suggesting the bondo would either shrink or show through a paint job.

I have no intention of learning to weld on a gas tank and I really don't want to drill a hole in it either in order to try to pull it out.

The dent can't be reached from the filler whole with a straight bar, but possibly with a curved specialized tool.

So in conclusion I feel like I have to either take it to a speciality shop and have them used a specialized tool (and experience) to work the dent most of the way out from the inside. IF I do that, then I'm not confident about how to finish it off. Will bondo work? I've never used any other material like the resin or kitty hair stuff I've read about.

I do intend to sand all the paint off, down to bare metal and repaint flat black. I even thought briefly about just saying F-it and leaving the dent and calling it cafe-character? But I'm sure I'd regret it later.

Any advice is much appreciate as always.

 

[SimJen]

I would push it out the best I could from inside (curved bar or something) then fill and repaint.
Alternatively, you could drill a tiny hole in the centre of the dent, attach a slide puller and pull it out that way, a blob of weld to fill the hole once you've finished.
Whatever, you will need some filler to get it sorted.

 

[80Bobr]

Another good option is to remove the fuel tap (to prevent damage) plug the fuel tap hole and put compressed air down the filler hole, the pressure in the tank should push the dent out then you can work with it from there, you'll probably have to use a rag to help plug around the filler hole.
I use this method at work to test fuel tanks for leaks and on the plastic tanks you can see them swell right up and hold pressure.

 

[roadahead]

I'm going run by Home Depot and see if I can find some type of curved bar that might work. I figure I'll give that a shot combined with some heat from a hairdryer.

As far as the compressed air goes let me ask you a question. The one concern that I've heard from several people is the possibility of blowing up the tank with too much hair. I don't have a compressed air tank anyway. So what do you think about using a bicycle pump? I have pretty good one they can put upwards 200 pounds of pressure into a bicycle tire. I wonder if that might work by going slow and steady.

My hope is that using a bit of both tactics gets the job done.

 

[SimJen]

I dont think compressed air will work as well as you think.
It has a few creases which will cause a lot more pressure to have to be used, not always a good thing!
A bike pump will just not work, you'd need to fully seal the tank, the caps usually aren't that great!
Just use a bar and push it out, knock any high spots back in, sand, fill and paint!
Oh yeah, also please be careful with the hairdryer! you are dealing with very flammable vapour!!

 

[Reinhard]

Dont drill holes in your tank. Take it to a body shop and have them pull it using a stud welder and puller. Or go to HF and buy a cheap stud welder and do it yourself. Pounding the dent out from the inside works too but really takes some finessing. You will never get the dent out to the point where filler isn't necessary. Filler is always necessary. Compressed air/heat won't work on that type of dent. The crease makes it stronger than teh metal around it.

 

[roadahead]

I dont think compressed air will work as well as you think.
It has a few creases which will cause a lot more pressure to have to be used, not always a good thing!
A bike pump will just not work, you'd need to fully seal the tank, the caps usually aren't that great!
Just use a bar and push it out, knock any high spots back in, sand, fill and paint!
Oh yeah, also please be careful with the hairdryer! you are dealing with very flammable vapour!!

Thanks SimJen. I figure you're right. What about the fill. Can I just use the regular everyday automotive bond that I have a little experience with or is something special need to really do the job right? I've just heard some concerns about the bondo shrinking and or showing through paint.

 

[SimJen]

any automotive filler will work fine, just be sure to get a good bond to the bare metal below.
Mix it correctly and sand it correctly, and let it cure fully before painting.
A good job will not be visible beneath paint!

 

[roadahead]

any automotive filler will work fine, just be sure to get a good bond to the bare metal below.
Mix it correctly and sand it correctly, and let it cure fully before painting.
A good job will not be visible beneath paint!

One more question...I think. If I stay away from the hair dryer completely (if that will indeed not help anyway) do I need to drain the tank of all gas and let it air out before I start prying on the dent with a metal tool?

 

[SimJen]

Its a good idea, you don't want a stray spark or anything!
When I work on tanks, I'm usually sitting down with the tank in my lap. You end up rotating it around all sorts of funny angles to do what you have to do!

 

[raf-1200]

Yes always a good idea to remove the fuel. Make it a lot easier to move it around, pry it out etc. don't be scared of filler. It's amazing how many bikes leave the factory floor with it already. Prying out will work if you have the patience. Be careful not to press down too hard around the cap lip as you can end up with a cap that doesn't seal. I've found a Teflon/plastic punch comes in handy when prying out just for hitting the high spots and releasing pressure. Yours has some fairly big creases though. One tool I came up with once was a mini jack made of threaded pipe with rubber ends. Was able to put it inside the tank, one end against the tunnel and the other against the dent. Then using a ratchet spanner I opened up the jack. That was a harley tank though so it was heavier gauge with a bigger hole. Forget that lol

 

[jsharpphoto]

I've used a blood pressure cuff to fix a tank. Deflate and roll up the cuff, insert it through the fuel cap and push it in the direction of the dent. Get it as close as you can and start pumping the bulb. The dent pops out, then you just open the valve and pull the cuff out.

 

[SimJen]

I've used a blood pressure cuff to fix a tank. Deflate and roll up the cuff, insert it through the fuel cap and push it in the direction of the dent. Get it as close as you can and start pumping the bulb. The dent pops out, then you just open the valve and pull the cuff out.

Sounds like a very good idea!

 

[RoadRash]

Tried that. All that happened was that it ruined a new cuff. Had mixed results with a heat gun and quick freezing with compressed air. Went through too many cans for limited results. Best luck I had was with my HF paintless dent removal kit.

 

[BoJordan]

Sorry for digging up an old post, but how did u go with fixing the tank? What did you do? I have a really bad dent on the top of the tank just in front of the seat where I'm pretty sure someone has made themselves infertile. The dent has created a pretty sharp crease and so I have experimented with a few strange methods, but nothing has completely pushed it out. I used a rod and hammer for a few hours the other day, did nothing at all. then I drained the tank and filled it with boiling water and sealed it up and poured more boiling water over the dents, this pushed it out quite a bit, but it is still not good enough. At the moment I have a hammer super glued to the centre of the dent and I am waiting for it to dry, apparently this is a good method for large dents, I just hope the araldite super strength will hold up for a good solid yank. I really don't want to have to waste my money on a panel beater

 

[roadahead]

Sounds like you're making progress. After several different attempts I decided to go in a very different direction. Of course my dent was in a different location but I decided to my lemons and make lemonade. I turned my accidental dent into the motivation to create hand dents up front and matching knee dents.

You can see the progression here: https://www.facebook.com/media/set/?set=a.10200310377974771.1073741826.1013679186&type=1&l=ad1605414f

Here is the finished product: