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I pressured my 1973 B50mx tank to try and pop out a few dents.
In hindsight, this was not a good idea!

I wasn't even up to 25lbs of pressure when the tank blew a leak at a seam. Dammit, ok, I can fix that. I unhooked the air and that's when I noticed the tank had deformed in a few different places. The worst is how it spread wider along the part that sits on the frame - probably 2 inches! Like spreading open a hot dog bun. That affected the top spine as well.

It's screwed.
I never could leave well enough alone.

Anyone have a good experience with replica tanks from India or anywhere else? I've read mixed feedback. Or do you have any other sources for a replacement?

I'm not building a show bike, so I'd just like something decent thay won't break the bank. For those of you that have an MX with the small tank and ride casually, would you put on a larger tank given a wonderful opportunity like I have? The thought has crossed my mind.

John

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For Indian tanks the source is important, some buy the seconds that fail and sell them on without the discount, Royal Choppers and Pummy8081 have good reputations via postings on here and elsewhere. If they don't have a listing for the tank you need but you know it is made in India just ask them.

Max pressure on any tank is 3psi just for leak testing, as you have found out more is not good.

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Originally Posted by kommando
Max pressure on any tank is 3psi just for leak testing, as you have found out more is not good.

facepalm
Oh man. I blew it, literally.

Thank you for the sources. I found a decent OEM tank on ebay for around the same cost as the import replicas, so it seemed like a safer bet.

John

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yes take care pressure checking tanks i took one to local radiator shop so called experts they blew it to 20lbs it came out looking like loaf of bread


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The tanks aren't cheap or common.
You could probably find someone to cut the bottom out and bash it back to shape, welding it back?

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If you do not mind the colour or larger size an '85 KX250 tank can be fit. Fits over the backbone. Not sure about length with a stock seat. I have a Suzuki 600. Plastic base far less prone to cracking.

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Originally Posted by AngloBike
The tanks aren't cheap or common.
You could probably find someone to cut the bottom out and bash it back to shape, welding it back?

Problem is that the aluminum has actually been stretched when the tank deformed. So attempting to return it back to original would be quite difficult. You'd likely have to cut open a number of the seams to try to get back into the correct shape. Perhaps some trimming, etc.. Paying someone to do this ( if said person could be found) would likely cost more than buying another used one.


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Originally Posted by EagleLakeWoodworking
I pressured my 1973 B50mx tank to try and pop out a few dents.
In hindsight, this was not a good idea!

I wasn't even up to 25lbs of pressure when the tank blew a leak at a seam. Dammit, ok, I can fix that. I unhooked the air and that's when I noticed the tank had deformed in a few different places. The worst is how it spread wider along the part that sits on the frame - probably 2 inches! Like spreading open a hot dog bun. That affected the top spine as well.

John

And I bet the dents were still in place shocked


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Not trying to be a spoiler (but probably succeeding!) you should not put more than 2-3 psi in a gas tank when leak testing it.
At the 25 psi you quote you have a lot of energy in that tank and if a seam suddenly split it could certainly do some damage to you--like blinding you for example.
I am posting this just to prevent anyone in the future reading this thread and trying to leak test at a higher pressure than 2-3 psi.

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After sleeping on it and giving it another look today, I realized it wasn't as bad as I thought. I did bust a seam, but I think I was over exaggerating the deformity. I had taken some pictures before engaging in this bright idea, and I compared the tank to those and found it wasn't too bad. I used some ratchet straps to tighten the sides in. I can't say it actually did anything, but it made me feel like I did something to correct this folly

This dumb ass idea was born from watching a guy on YouTube getting dents out of a 2 stroke exhaust. He pressurized to 150psi, heated up the dents and they popped right out.

I'm sure some one here will enlightened me on the fundamental difference between my attempt and that exhaust video. Beyond the obvious difference of softer aluminum versus steel, why is that exhaust capable if so much more pressure?

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I've heard/seen someone who used serious air pressure on a trail bike tank (j*p*n*s*).
He said it spread its wings like a butterfly !! Oooops.
The exhaust can only expand outwards, so you are on much safer territory there.

btw, it used to be a common cheat in production bike racing to use OIL pressure to gently expand a tank,
to achieve slightly more volume. This is difficult to detect, unless you have an original tank side by side.
They generally clamp the tunnel bit (over the frame) together, so it doesn't do that butterfly wings trick.

Oil because - unlike air - as soon as you reduce the pressure the expanding stops.
Whereas air can compress, and once any expanding gets underway it can continue,
sometimes quite a ways after you turn the pressure source off.

Compressed air stored in vessels can be dangerous if it springs a leak - use at your own peril.

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Originally Posted by EagleLakeWoodworking
Beyond the obvious difference of softer aluminum versus steel, why is that exhaust capable if so much more pressure?

Shape, the tank is a U and the pressure wants to unU the U and make it --- .

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Exhaust pipes, even two strokes, are usually round so the radial force is equal all around. If you did that to a B50 box muffler it would do the same as your tank.

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Thanks kommando, that's a giod
Originally Posted by kommando
Originally Posted by EagleLakeWoodworking
Beyond the obvious difference of softer aluminum versus steel, why is that exhaust capable if so much more pressure?

Shape, the tank is a U and the pressure wants to unU the U and make it --- .

Thanks Kommando, that makes perfect sense and that's pretty much the way it went for me

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One way that sort of works is to heat the dent with a hot air paint stripper, then apply ice or preferably dry ice to the dent..
Shrinking the centre of a hot dent rapidly causes the dent to pop out (in theory)

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I fixed the dents in a couple B44 & B50 tanks with a pair of long vicegrips commonly used by welders. I put rollers on the ends of the arms. rounded one just big enough to go through the gas cap, the other was straight, making an English wheel thing. I couldn't reach all of the dents but got the big ones up front. heres a link to the tool I used as a base

[Linked Image from i.ebayimg.com]


one secret to make it easier is to anneal the aluminum. mark it with Magic Marker or acetylene smoke, then heat it until the marks burn off, douse it with water

Last edited by Mitch; 05/02/21 10:08 pm.
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Originally Posted by Mitch
I fixed the dents in a couple B44 & B50 tanks with a pair of long vicegrips commonly used by welders. I put rollers on the ends of the arms. rounded one just big enough to go through the gas cap, the other was straight, making an English wheel thing. I couldn't reach all of the dents but got the big ones up front. heres a link to the tool I used as a base

one secret to make it easier is to anneal the aluminum. mark it with Magic Marker or acetylene smoke, then heat it until the marks burn off, douse it with water

Thanks Mitch! I'm going to give this a shot. My dents are right up front where the fork hit the tank.

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Originally Posted by EagleLakeWoodworking
This dumb ass idea was born from watching a guy on YouTube getting dents out of a 2 stroke exhaust. He pressurized to 150psi, heated up the dents and they popped right out.
Those two smoke guys are magic. Somehow they can also remove dents from pipes by filling with water and freezing them. Maybe they just say that to get dupes like me to try it?
I never could figure out how to seal an exhaust pipe well enough to hold 150psi. I can't even find clamps that hold my mufflers on tightly, with or without beer can shims.


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It'd be like chroming old exhaust bits ?
You WELD up the ends
And then grind them off later.

Or it could go off like a rocket, and take you with it !

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The way to do this with an alloy tankwithout making internal formers is as follows
1) make up a packing piece that is a tight fit between the two sides then strap the tank to it
2) block off the fuel outlet with a plug
3) fill the tank with water & block off the filler cap with cling wrap ( pallet wrap is better ) then close it
4) Hold th tank so the dent is upper most
5) Heat the edges of the dent going round & round, this will take a long time, probably an hour or more

As the water gets hot it will expand and because the dent is hot and a little softer than the rest of the tank, it will roll the dent up.
As this happens move the torch to just where the dent curls in.
When finished set it aside till it is stone cold before opening the cap.
Once the dent is out , it will have stretched the alloy a bit which can be heat shrunk, but you need to be really careful that you don't burn a hole in the tank.
Any one who makes aluminium car & bike parts can do that bit and you are probably better off sending that bit out.
Check out Kent White's site TM Tech but be careful.
Sheet metal forming can be very addictive
If you have the time, take the tank to one of his courses so you can pick up a new skill & get the tank repaired at the same time

Last edited by BSA_WM20; 05/03/21 8:27 am.

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There is a guy that can remove those dents. He isn't quick or cheap but he is very good. His name is Joe Abbate and he runs a shop in California called Cycle Therapy. He specializes in vintage MX restorations and alloy tank repair. He also runs a facebook show called Vintage Motocross Q & A.

He removed several dents from my B50 tank. I had the busted fork stop dent, a shallow knee/leg dent, several small dings and deep scratches and some weird crosshatching on the right front.

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