gas tank finish work

[aubicon]

i have finished plating togather a motorcycle gas tank and after block sanding i have found a few low spots. is there any way to raise these, its a sealed tank so i cannot get a dolly on the back of the pannel. i am trying to avoid any kind of filler lead, plastic, exc.. any sugestions?

[kustomizingkid]

I'm guessing the lows are stretched so hit them with a shrinking disk and they should slowly pull themselves up or you could try a dent puller with spot weld on pins to pull with.

Brandon M.

[87gtmustang]

Yes those may work, but only if you haven't had gas in it yet. Its common knowledge, but I just figured it was better to be safe than sorry, so I mentioned it. Also depending on were they are and how strong of a dent they are, you may be able to push them out with some makeshift prybars sticking them throught the gas filler opening. Like paintless dent removal, but still needs to be painted. lol

Brian =)

[jimmib]

Depending on the size of the dents and how many, you can also try an O/A torch to heat the dents and then rapid cooling with wet rag, ice, etc.

Jim
http://www.diybikes.com

[Wray Schelin]

i have finished plating togather a motorcycle gas tank and after block sanding i have found a few low spots. is there any way to raise these, its a sealed tank so i cannot get a dolly on the back of the pannel. i am trying to avoid any kind of filler lead, plastic, exc.. any sugestions?

First locate the low spots with a magic marker. Next make a dolly out of a 3/4" dia steel rod about 20" long. Bend the rod so it looks like a J shape and make the short end a low crown dolly face. Next feed that into the filler hole of the tank and get under the lows spots. Using a slapper hit over the low spots until the J dolly raises them up .

Takes patience plus you might need several special dollies but you can work out those lows. Next time make sure they are gone before you weld your tank bottom on.

[goleafsgo_12]

I understand the concept of putting the J into the tank, but how would hitting the the low spots with a slapper raise them up to be level with the tank? I assume low spots mean dimples into the tank, and not raised spots the protrude from the tank, correct?

Ryan

[Wray Schelin]

Hi Ryan,

Low spots in any panel are raised up by hitting the outside of the panel inward. The trick is to have more force against the low spot than what you apply with the slapper. Lots of little slaps on the onside of the tank brings lows up to the surface if you have your dolly positioned correctly.

Taking dents out from the back side of a panel by hammering or roughing out with a dolly is OK for roughing only, fine work is always done from the top side of the panel.

Think of how a planishing hammer works, or an English wheel. They work by forcing the top die over the bottom die with the metal in the middle.

[anders nørgaard]

Hi Ryan,

Low spots in any panel are raised up by hitting the outside of the panel inward. The trick is to have more force against the low spot than what you apply with the slapper. Lots of little slaps on the onside of the tank brings lows up to the surface if you have your dolly positioned correctly.


Hi Wray,
Isn't this the way, that's called "off dolly"?

[Wray Schelin]

Hi Wray,
Isn't this the way, that's called "off dolly"?

Hi Anders,


A low is either an area that needs to be stretched or it is a area that needs to be shrunk.

How, do you tell the differance?

Lows that need stretching are underdeveloped areas that need stretching for them to be able to rise to the common surface level. To fix them you need to hit on dolly which adds the needed surface area in that low spot to allow it to rise to the common surface.

Lows that need shrinking are overdeveloped spots that have too much area. To identify them you will notice that they have a high zone on either side of them. In effect they are part of a wave of the surface. There is too much surface area in one spot for it to be the desired shape so it takes the form of a wave. First you need to remove the wave by rearranging it into a swell. You do that by hitting off dolly with the dolly on the low. The result will be the desired rearrangement. Which means the wave will transform to a swell. After you have the swell you then shrink it to the desired shape's true surface level with a shrinking disc.

This might all sound like either rocket science or hooey, but it is all very easy to understand when demonstrated. It is one of the fundementals that I teach in my sheetmetal shaping classes.

All sheetmetal shaping is about understanding lows and highs and how to arrive at a desired shape's common surface.

[Boogiemanz1]

That is an excellent explanation Wray. Iwould like to interject that a lot of newcomers don't have any concept of the amount of time that a seemingly simple job can take.

If you are a student of metalwork/metalfinishing the first thing you will have to learn is patience. Wray for instance could take this tank, equipped with handmade tools that he has built along the way, and make short work of it.

Most novice, and a lot of people that have been at it a long time will take the tank and try several options out of inexperience, maybe find a way to get it done, maybe not. If they are motivated, and spend the time they will be rewarded with a great panel and a kernal of experience to add to their knowledge. Please don't misunderstand, there are several ways to do this, but for most they all are a learning experience. Taking instruction from someone like Wray that has LOTS of experience will help you understand what has happened to the metal, how to correct it, and how to apply that to other situations.

We have some of the best metalmen anywhere on this board. Some of them are beginning to offer classes. These are guys that do a wide range of metalwork and have lots of clients that keep them busy. All of them try to take time to help the rest of us on this board. I would like to see some support for their classes from members of this board who are seeking to shorten their learning curve............john

[anders nørgaard]

Hi Wray,

Thanks for this great explanation.
I got it (Ithink) <LOL>

[jvo]

Wray, I don't think I've read a better explanation than that anywhere. Its taken me a lot of years to begin to understand this, and you just summed it up in a paragraph. Thank you. John V.O.

[Tisdelski]

hi wray, your explanation of how to determine lows that need to be stretched or shrunk really opened my eyes. i have been shrinking the highs on the outside of the lows first thinking that they are holding the lows down. i can now see that its easyer to shrink one smooth high spot compared to shrinking two high spots and then having to stretch the low. thanks gary

[aubicon]

thanks Wray, now i know where i will begin. i picked up a shrinking disk last week and havent had a chance to use it so here we go. i am interested in these J shaped dollys that someone mentioned, i think that i understand the basic consept, can anyone give a more precise example or is it just that simple?

[jibbyjammin]

you know, you can always go back to the old fashion way to raise dents.
You could use water in the tank and freeze it. This way the water will freeze and push dent out.
The other way is with air. If you place an air bib on the tank cap, then place it on the tank, proceed with air untill dent pushes out. Just be carefull and pay attention, not to place yourself in harms way.

[Peter Miles]

Do be careful pressurizing fuel tanks.

When I first started playing around with homebuilt airplanes 25 years ago, this was something that was pointed out repeatedly. It is very easy to delaminate a composite wing with "wet-wing" tanks with just a very few pounds of air pressure while checking for leaks.

A 4' x 8' sheet of metal is 32 sq. ft. One sq. ft. is 144 sq. inches. 32x144 = 4,608 sq. in. in a 4'x 8' sheet. 5 PSI across that entire sheet is 23,040 pounds of force. That is a lot of potential energy.

If that 4' x 8' sheet were folded up as a 4' x 4' x 2' rectangular tank and you pressurized it to 5 PSI, you have that same 23,040 pounds of force looking for some work to do or someplace to escape.

Peter Miles

[Boogiemanz1]

Air pressure or freezing water in a tank sounds like a real good way to deform a tank or get hurt. I believe this is a tank that was just welded together, and warped or still had low spots. Working the metal is the way to go in this case. I blew a fatbob out of porportion once just trying to leak check it...................john

[FriarTuck]

Hydro-ing tanks is safe and more effective. Water won't compress, so if the tank does lets go, all you get is a water leak. If a tank pumped up with air lets go, all you get is scrap. There is a lot of stored up energy in compressed gases of any kind. Fill the tank with water, connected to a head-tank of some type, or just lightly pressurize the tank that is full of water. That cuts down the room for air pressure energy build-up. Be safe, not sorry!

[Ernie Ferrucci]

No way on using air pressure to remove dents!

Hi Aubicon:
(I don't know your real name, they guys on this site like real names)
It is not worth the risk of injury or wrecking the tank.
I agree with John and the others, work the metal.

You only need 1 or 2 psi to test a motorcycle tank for leaks. I have no reason to doubt Peter's calculations, that's a lot of force!
I seal mine up the best I can, put a whip of hose on a barbed fitting, charge the tank with air and hold it under water. Bubbles = Leak. I like to hold it under for a while and watch the welded seams. Sometimes you will get a tiny slow forming buble, that's a leak also. Mark it and weld it. Gas will find it's way through the smallest pin hole. Those are the ones that buble the paint even months later. After the tank passes the test I like to clean the inside and coat with fuel tank sealer.
John: I know those fatbobs aren't all that thick in the metal department, worked on a lot of them. Cut up a bunch of sportster tanks also, same gauge.


Good luck
Ernie Ferrucci

[jvo]

Wow, Peter, thanks for that. I totally understand the principle, but until you laid it out on paper, I never really thought about the total force involved. I will have over 10,000 lbs on my gas tank, that I'll be welding up next weekend. Last one I welded up for a friend, we put 5 psi, could maybe have been 10, I'm not really sure, and used a little water, with dish soap in it, to find any leaks. This time, I'll be really careful to only use 1 or 2 psi. John V.O.

[Peter Miles]

5 PSI will 'float' a steel plate 1.76" thick. We put 65 PSI into a bicycle tire so it must be safe, right?

Another interesting area of physical phenomena associated with pressure is the overpressure effects associated with large weapons. I'm sure that most of you have seen the sequence below in one or more movies.

http://www.radshelters4u.com/exphouse.gif

That was external overpressure from a nuke several miles away, not something exploding internally.

One or two PSI of overpressure can cause substantial damage. I just did a Google search on overpressure and got the following from www.radshelters4u.com (http://www.radshelters4u.com/), which took the originals from FEMA sites, etc.
http://www.radshelters4u.com/blast1.jpg
You can see that 5 to 10 PSI would be pretty catastrophic. A tank pressurized to 10 PSI that let go quickly and completely in an enclosed space could cause some significant damage.
http://www.radshelters4u.com/blast5.jpg
That 295 MPH wind blast associated with a 10 PSI overpressure would also be a bit brisk.

Obviously a one or two-gallon tank is going to have effects limited to a rather small area. The energy involved should still be respected, however. You're likely to be pretty close if it should fail.

[Kerry Pinkerton]

Back in '81, I was offered a job with a contractor supporting the space shuttle program office at Marshall Space Flight Center. Turned the job down but one of the things they were doing stuck with me. They had these OLD Modcomp real time computers tied to tons of data collection devices all over the external tank. The purpose was to pressure test the tank. This was actual flight hardware.

I was surprised when they told me that it to some number of days to fill the tank with water. "Water?" I asked. "Why not just air?" Because air stores energy and water doesn't. If the tank failed with whatever PSI of air they were testing to (I don't remember but I seem to recall it was a LOT) it would level the better part of the center......

[Gene_Olson]

A split rim in a cage has killed more than one. at 100psi.

and speaking of bicycles.

The old balloon tires were inflated to 32#, average 10 speed 65#, racing tire 110#

When I was about 10 we went to visit some friends and I borrowed Bertha's ballon tire bike,
unfortunately the rear tire was low,
fortunately there was an auto repair shop next door with an air hose.
I knew how to read an auto fill pressure gauge.
Unfortunately their in line tire gauge was broken.
Fortunately the tire held as I inflated it to what the gauge said was 20 psi and wouldn't go higher, bummer. ( actually line pressure 115psi)

I got on the bike and started riding back to Dammans.
Fortunately there was a smooth sidewalk,
unfortunately there was a big elm root that had lifted a section about an 1 and a half inches.
When the rear tire hit the bump, it exploded and I and the rest of the bicycle did a neat pivot around the axis of the front tire and landed facing backwards and upside down on the sidewalk.

Fortunately I survived.
Dad offered to pay Fritz for the blown tire, but he declined, something about it being the worth it, the most entertaining thing happening around there in a long time.

at the time I failed to see the humor in the situation.

One of my tire places has a sign on the wall, "remember, there is enough energy in a Loader tire to send a 12" bowling ball 20 miles"

G

[Gonejunking]

I use a bicycle air pump and a spray bottle with dish soap and water.
1-3 psi max, soap bubble's stay put unless it's a big leak.

We do blow test's on airplane tube's, at 5 psi, we can tear the seat's out of the plane!

Just my 2c

[Ernie Ferrucci]

Hi Steve

You will find some useful information here: http://www.metalmeet.com/forum/showthread.php?t=3007&highlight=pressure+test discussion on testing begins on page 2.
I sure there are other threads if you use the search tool.

No way on using air pressure to remove dents!

Hi Aubicon:
(I don't know your real name, they guys on this site like real names)
It is not worth the risk of injury or wrecking the tank.
I agree with John and the others, work the metal.

You only need 1 or 2 psi to test a motorcycle tank for leaks. I have no reason to doubt Peter's calculations, that's a lot of force!
I seal mine up the best I can, put a whip of hose on a barbed fitting, charge the tank with air and hold it under water. Bubbles = Leak. I like to hold it under for a while and watch the welded seams. Sometimes you will get a tiny slow forming buble, that's a leak also. Mark it and weld it. Gas will find it's way through the smallest pin hole. Those are the ones that buble the paint even months later. After the tank passes the test I like to clean the inside and coat with fuel tank sealer.
John: I know those fatbobs aren't all that thick in the metal department, worked on a lot of them. Cut up a bunch of sportster tanks also, same gauge.


Good luck
Ernie Ferrucci

Hope this helps
Ernie F.