Topic: To fix a dent or two

[Kong]

First you got to have a dent.  Fortunately I had several of them all in one side of the same tank.  Actually I had more of them on top and the other side too, but they were small.

The big dent is actually two dents, a smaller crease at the bottom and the larger caved in area.  There is also a smaller creased dent forward of the knee-dents.  So this is where we begin:

[Kong]

I'm going to remove the dents by blowing them out with air pressure. To do this I will take a blow-gun with a rubber and masking tape tip and blow air into the tank.

The tool:


I take the blow-gun and  push it up into the petcock hole and while concurrently holding my thumb from the other hand over the cap vent blow just a bit of air into the tank.  It takes very little air, and in fact if you blow too much into it you can distort the tank beyond repair.

The Process:

[Kong]

And after about 5 seconds it goes Ping!  And presto, its dent-be-gone

After:


Notice that the sharp creased dents did not blow out.  They will be fixed by conventional means.  Fortunately they are accessible from the inside with shaped bars, so I'll push them out and finish with a skim coat of flowable filler during the prep process.  But the air takes the big dents out completely and there are no remaining hammer marks.  You still have to flow it in the prep process to make sure there is no wave from metal memory, but that is really something for another post.

[HondanutRider]

Neat tip!   How much air pressure do you recommend?

[Kong]

Very very little.  I run my pressure at 100 pounds in the line, which is pretty common.  I blow into the tank in short bursts, maybe 1 to one and a half seconds at a time.  Keep in mind, you can really screw up a tank doing this if you blow too much air into them.  The worst one I ever screwed up was a Honda Rebel tank this past summer.  It had big rolling dents in the top and side and I got them all easily, but in trying to get one last one that had a little too much crease in it I blew the entire tunnel out about a half inch and the tank wouldn't go back on the bike until I took a 2-pound hammer to it and beat the living bejesus out of it.

Be careful, but don't be afraid.

[scottly]

Be VERY, VERY careful doing this!! I ruined a tank doing this, many years ago!!

[Blueridgerunner]

+1  me too 

[mrbreeze]

Have you heard about filling with water and freezing? I read somewhere about it but don't recall if it was the best way to go.I currently have my 75K that I've had since 2000 and have a straight tank but very rusty on the inside. I bought another one off of ebay that is clean inside but has dents. Last April I got 2 parts bikes (71K&74K) and both of those tanks are pretty much junk (rust & dents). Just for sh!ts & giggles....I may try your compressed air trick on one of those tanks.

[Kong]

With the water thing, aside from how long it would take, is really pretty uncontrollable.   The way I've done this the several times I have is to make sure that I use my finder to plug the vent, no matter where its located.  For most tanks, with a large vent, that all by itself limits the pressure that can be built up.  However with a tank like the one in the pictures, where the only vent is in the cap, it is very small and so even holding your finger over it allows large pressure to be built up.  Ya gotta be careful.

[lone*X]

I completely ballooned a 750 Virago tank in the early 90's doing this.  Maybe the Yammy tank was thin metal but it blew up like a balloon and I was using a bicycle pump and did not have that much pressure in it.  Had to buy another tank.  Too many years back to remember how much pressure but told myself then that in the future I would not try this again.  Since then I had very good results on a tank by tacking short rods around the low areas of the dent and using a slide hammer to pull the dents out, a little at a time and moving around the dent.  Just don't use too much amperage and burn holes in your tank and be careful when you grind them flat afterward.  You only need to get most of the dent out so the filler is kept as thin as possible.  Oh, and I filled the tank with water before welding to it.  Can't be too careful.

[AndrewCola]

Cool. I'll try this on my cb550

[verslagen1]

I've thought of using this technique for removing some dings.

I want to pack the areas with no dings with non hardening clay and placing the whole thing in a box for support.
areas with dings can be left hollow or with a shallow air pocket  for the ding to over expand to.

I work in a shop where high pressure ducting is made and we test such ducting in a water tank.
occasionally, we find a defective one and we'll need to refill the tank.  don't repeat DON'T use high pressure air in the tank.  If it get high enough, it's like a bomb.  That tank will tear you to pieces.

Let's say you're trying to push out a 3x3" square dent, that's 9 sq. inches.  At 1 psi that's 9lbs, at 10 that's 90lbs, at a 100 psi that's 900lbs.

Now that's considering I've isolated the dent in the clay packed box.  A tank out in the open air?  it's the whole tank you're applying the pressure to.