Knee dent nightmare

[Danilo80k]

I have a 1980 CB750K that I'm working on. This is my first motorcycle and my first time really doing anything mechanical. I'm turning into a modified cafe/brat thing. So the past couple of days I have been working on knee dents. Tonight while I was banging them out I noticed it was getting wet. I thought it might have been mineral spirits from when I was wiping off the dust. I wiped it, hit it some more and it got wet again. I was wearing a mask so I didn't smell the gas then it dawned on me. The tank has a crack where the badge originally was. Same thing ended up happening on the other side. It had some kind of filler in it from the previous owner taking the badges off when he painted the bike. I either sanded through the filler or cracked the tank by hitting it. So question number one, How do I fix the crack? I've read things that say to solder it. That sounds the best to me but I'm open to suggestions. What kind of solder would I use to do that? The crack is about 1/4" long.

Part 2: After all that stuff happened I went to put the tank back on the bike to see if how it looked on the bike and I couldnt get the tank to go on. The spacing between the sides of the tank got pushed in just a little from me putting in the knee dents. What do you recommend for bending it back out? I have a little piece of wood between the things that hold on to the the rubber parts on the frame to try to push it out a little.

Otherwise the dents actually look good to me. I was nervous to try my hand at metal work but it pretty much shaped it's self. I just need to get these problems solved ASAP so I can get my bike painted before I have to go back to work. I'm on leave right now. Thanks in advance.

 

[Sonreir]

Did you try to pound dents into the tank while there was still gas in it?

And lemme guess... you pounded the dents in using a claw hammer?

Regardless, it's time for a new tank. If it's bent bad enough that it won't even fit back on, then you're damaged it beyond the point where repair is going to be cost effective.

 

[Danilo80k]

No I dumped as much of the gas as I could get out. There was just a little bit sloshing around that I couldn't get. I definitely didn't use a claw hammer. I do research before I do anything I don't know about. I used a teardrop mallet and a tiny ball peen for the corners. I can get it back on the frame by just pulling out on the sides some but as soon as it gets over the pipe it bends back in a little. It's just barely out of alignment. I'm probably making it sound worse than it is.

 

[gijoe13844]

If you moved the metal enough to make it not fit then it is worse that you think, those tanks have quite a Bit of slop in tolerances to where even small moves wouldn't make it not fit.

The crack needs to be soldered and if you don't have the knowledge or skills to shape the metal without damaging it I doubt you would be able to pull off a sheet metal solder job. Not a knock on you as a person just the way it is. Very difficult to do right and safely. And may not even be possible if rust has undercut the crack area.

Buy a new tank. If you must have knee dents then do it right or have someone who shapes metal do it for you.

If you still think it isn't as bad we are assuming then post an actual photo.

 

[Reinhard]

Get a caswell kit and follow the instructions for repairing hole You don't need a new tank. Solder won't work. Welding or brazing would. Get a Caswell kit.

 

[gijoe13844]

Get a caswell kit and follow the instructions for repairing hole You don't need a new tank. Solder won't work. Welding or brazing would. Get a Caswell kit.

He is correct...brazing is what I meant...not like electronics or pipe soldering. However I would not trust a caswell or any sealer to fix that size of a hole.

 

[Reinhard]

The caswell kit comes with fiberglass tape to repair holes from the outside before sealing the inside. It works. I've fixed holes the size of a dime with their kits.

 

[VonYinzer]

Let's all argue about it.

Buy a new tank and do the job right. Patching up improper work is a cop out.

 

[SONICJK]

I don't see where the improper work comes in?
He hit It a bit hard.

I've had the same issue with a 350 tank I dented like this, just pull the sides apart until it fits back on.

As far as the leaks I'd find a friend to weld them up. (After filling the tank with water to remove all traces if gas, then dump the water and rinse again with gas and a little two stroke oil to keep it from rusting

 

[Reinhard]

I'm not arguing. I'm simply stating what has worked for me. And I don't agree that patch work has to be shoddy. I don't throw away forty year old tanks that can be saved.Epoxy technology has come a long way. Anyway, Merry Christmas all.

 

[gijoe13844]

I don't see where the improper work comes in?
He hit It a bit hard.

I've had the same issue with a 350 tank I dented like this, just pull the sides apart until it fits back on.

As far as the leaks I'd find a friend to weld them up. (After filling the tank with water to remove all traces if gas, then dump the water and rinse again with gas and a little two stroke oil to keep it from rusting

Just my opinion...that comes from designing and trouble shooting tools that stamp motorcycle gas tanks...that if you are causing that much stress that you are screwing up even 40 yer old tank tolerances then you are very likey stressing the metal elsewhere. By bending it back to fit the stress gets even worse.

It may be ok....I dont have any photos to go off but combine that with the fact there are cracks in the tank already he is asking for way more trouble down the road. Von is right...new tank...do it right.

 

[Reinhard]

I don't agree that the correct way to fix something is to throw it away and buy new. We are not talking about a cracked block.

 

[gijoe13844]

I don't agree that the correct way to fix something is to throw it away and buy new. We are not talking about a cracked block.

I would be in agreement with you if it wasn't the most mass produced motorcycle in history.

 

[SONICJK]

I would be in agreement with you if it wasn't the most mass produced motorcycle in history.

It's not.

But if he wants knee dents he's doing to do the same thing again so what is the point of buying a new tank when the same thing is going to happen again? It's not like its load bearing. Fix it and run it I say.

 

[Danilo80k]

Here is a better picture of the tank. It's somewhere along the red line. I don't even think it spans the length of the whole line I drew. As you can see there was some kind of filler there. That's why I'm not sure if I just hit it too hard or I sanded through it or I sanded through that filler.

 

[Ringo]

No need to freak out here, or go to extremes people. Let's start from the beginning...are you going for a show bike?

If so, buy new and get help.

For everything else, work it until it fits the frame again, then line it with caswell OR take it to a good welder, then fill/smooth the uglys. Simple eh?!

Sheesh you guys.

 

[Danilo80k]

No need to freak out here, or go to extremes people. Let's start from the beginning...are you going for a show bike?

If so, buy new and get help.

For everything else, work it until it fits the frame again, then line it with caswell OR take it to a good welder, then fill/smooth the uglys. Simple eh?!

Sheesh you guys.

She definitely isn't going to be a show bike or I wouldn't be doing it myself. It's just something I can be proud that I did. I have a friend that is well verse in everything mechanical. He said we can braze it.

 

[SaltCityCafe]

That's something that isn't mentioned often. After pounding in the knee dents don't use any pneumatic or electric grinders or sanders or anything on those areas. You have stretched and thinned out the metal so its really easy to sand through it.

I just did mine too and had no issues. The key is patience and hit with firm blows don't wail on the thing. after you have them pounded in and shaped use paint stripper to take off the paint then hand sand it to get the remaining stuff off. It takes a little longer but its safer and the results are the same.