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MG MGB Technical - Welding distortion

My ’74 mgb roadster was originally US spec. So it had the cut away scuttle panel. I am in the process of converting it to RHD steel dash.

I cut the old scuttle top between the air vents and where the screen sits. Unfortunately I rushed my welding a bit and have ended up with rather more distortion than I would have hoped. The distortion is concentrated in 2 places, in line with the vent slots on both sides. It has sunk rather than risen.

What is the best way to deal with this? The idea I had was to put a jack and a block of wood underneath and try and jack it out but I think it might just spring back when I remove the jack. The other idea was to make up some bracing pieces to hold it in place from underneath. These would conflict with the air funnels though. Perhaps I should just use a bucket load of filler.

If I was doing this again I would change the whole scuttle panel. As it is I cut my insert piece from a new panel.
David Witham

David, you can try to raise the areas but they will both probably pop up and then stand proud. The metal has expanded from the heat and really needs to be shrunk. There are several methods to shrink areas in body panels, I suggest you find a good 'old school' type body man for advice. Filler can be used, I have done that in the past, but from my experience I now would recommend having a body man take a look first.
Bill Young

David, It sounds to me like you might need to "shrink" the metal a bit. I know, "what???". To shrink the metal, start by heating a small spot (about 15mm) to a dull red with a torch. When hot,the metal will rise or fall a bit, place a metal working dolly over the low spot in the metal and lightly hammer the "high" spot down a bit, then quench with a water soaked rag. The fast cooling will help to draw or shrink the metal. Do a small spot at a time until the area is flat. If you happen to shrink the spot too much, it can be stretched again by hammering on the spot with the dolly directly below. Do this cool. You might want to see if you can find a book on body/metal working techniques.
Good luck, The MG Doc
John

David:
Shrinking sheet metal is relativly easy it is not a black art.
Heat a small area to cherry red (about 3/4 inch across) just off center from the lowest part. Now cool rapidly with a wet cloth. You then place a dolly under the spot and hammer lightly on the top with a body hammer. Hammering is essential to shrink the metal. Just repeat the process around the low spot.

Weld two pieces of scrap together and prictice a bit first. You do not need an oxy. welder to heat you can do it with a propain torch. Try it you have nothing to loose as you neet to fill the area anyway.

Godspeed in Safety Fast
Jc
John Crawley

They say heating and quenching works to shrink metal, though I've never tried it. If an old panel beater can't set you straight, you could make a cut with a thin cut-off wheel perpendicular to the high spot, this would relieve the tension. Weld it back together, smooth and lead any low spots.
Kemper

Thanks for your answers. I have a few questions about them:

John (MGDOC), when you say high spot and low spot do you mean either side of the sheet in the same place (what is high on one side is seen as low from the other) or do you mean offset from each other across the profile of the dent (so high and low as seen from the same side)

John Crawley, when you say body hammer do you mean one of those with swirls on the face of the head. (perhaps the head rotates). I weld with a mig my gas torch is one with disposable cans that I use for plumbing, do you think it will do?

Kemper, when you say perpendicular to the high spot are you assuming a high spot that is a sort of ridge and making a cut across it so the cut is perpendicular ( at right angles) to the line of the ridge. In my case it is a dip but I assume for this purpose a high spot is a low spot viewed from the other side.
David Witham

Yes, I was thinking in terms of a fender flare, where the edge is warped and you can section it like a pie. If the high spot is in the center of a panel (sorry, I'm not specifically familiar with the conversion process), then I'd try heating it. A propane torch will get a small area hot enough, but might not work to keep a large area hot enough.
Kemper

David, I had to do the same as you. Convert my US car over to a steel dash so I also replaced the scuttle. The same thing happened to me, it is now a little dipped in. I haven't tried to fix it yet so I am keen to see how you get on!
Simon Jansen

Simon, I have had another look at the pictures on your site. What you have done is similar. The distortion does not show in the pictures.

Kemper, you "cut to relieve the tension" approach appeals to me as I have not done any panel beating in the past and don’t have the tools. My main concern is the danger of further distortion when I weld up the cuts!

Looking at the distortion again last night I started to doubt if shrinkage was what I want to achieve. The distorted area should be a convex curve when viewed from above. It has sunk so that it has flattened out the curve. So it may have already shrunk meaning that I need to stretch it.
David Witham

Welding shrinks the metal. You need to stretch the area a little to get it back right. I can't really picture what you're describing, but can you do a little on-dolly hammer and dolly working in the area? You should be able to pick up a hammer and dolly fairly inexpensively.....

How did you weld it? Gas? MIG? If the latter, it might be a little more difficult as MIG weld beads are hard, and MIG welding seems to harden the metal. If it's not too far out, filler might be the way to go.

Keep us posted!
Rob Edwards

You've got it backwards Rob. Welding (heat) stretches the metal.

David I had a go at this last night with a midget hood. A propane torch will work fine. In my case, cherry red wasn't necessary to get the metal to expand. It appears to make the problem worse when heat is added, but once cooled with a wet rag, it will shrink. Good luck.
Kemper

Torch method for shrinking should work well for what you are describing. I have an Electric "Stud Gun" that welds small copper rods to pull out dents. It also has a shrinking tip on it. Probably more than you want to spend (around $100 on E-bay), but if you have any dents you need to pull, these units are invaluable. Takes the work out of pulling dents and you don't have to drill holes for the old screw puller.

Type stud gun in your search engine
Bruce Cunha

David and Simon,
I did just the same as you fitting a UK steel dash on a 74 California car.
I tried the "shrinking" techniques above but ended up "slicing" the metal in two places with a cut off wheel,front to back, from the bonnet to the dash, to relieve the tension/warpage and then SLOWLY tacking it all back together.
Regards
Tony
Tony Bates

A ball peen hammer and a propane torch sould be all you need to get it close. Heat the low side - you should be able to watch the metal expand toward the heat. A few taps with the flat end of the hammer will raise the spot where it needs to be. If you have "tin canning" where the dent will pop in and out, you can tack weld a big washer on the back side to prevent the popping, or you can keep working on shrinking the metal until it stops. Next time only weld in very short increments - less than an inch at a time. That will prevent the heat buildup that warps the metal.
Jeff Schlemmer

Sorry Kemper, you've got it wrong. Welding shrinks the metal. As you heat it, it expands and becomes more malleable. The expanded metal has no where to go, so it becomes thicker. Then it starts to cool down and it becomes less malleable faster than it shrinks, so the thickness becomes fixed. End result -- shrinkage.
Rob Edwards

Not to get in a semantics argument, but note the word “heat” in parentheses. Welding, i.e. heat stretches the metal. Rapid cooling can shrink hot metal. That’s why quenching works, but the end result of welding isn't always shrinkage. It can also work harden, change the temper… Not that any of that’s relevant here.
Kemper

David, I live in St. Louis, I know John, MGDOC, his bodywork is some the best I have seen, I have no business relationship with him at all but I would trust anything he says. He is the only person I know who can cut two wings/fenders in half then weld the parts back together again invisibly.

I would guess he means high and low spots when viewed from either side of the metal.
The Wiz

http://www.weldingfaq.com/distortion.asp

"One of the major problems that one encounters after welding is weld distortion. All welds will shrink as the weld metal cools from red heat to ambient temperature."

HTH!
Rob Edwards

There is a couple things I would recommend. First, Eastwood offers a putty compound that you form around the subject area to isolate and confine the heat generated from welding.

Second, there is a product called "Cold Fire" that is amazing stuff. I saw the host of a tv show actually place his hand on a torched rag after spraying it with this stuff.
I would use this on the subject area to keep the heat from even building at all. The putty I mentioned earlier will serve as a safety barrier.

http://www.firefreeze.com/site/page8.cfm
Boris

Using the above products together should save your body panels, your paint and the additional work that comes with it.
Boris

I forgot to add that there is also a welders trick or technique that would reduce the number of breaks you will need to take between welds.

After welding a 1" stitch, stop and skip over and beyond where you started. Run the next stitch toward and ending where the last one began.
This gives the beginning end of each bead time to cool.
Boris

Thank you everyone for sharing your knowledge. I was away over the weekend so I have not yet been able to try anything.

David
David Witham

David, When I say to place the dolly over the low spot, I mean to sort of bridge the low spot, It doesn't matter if its on top or bottom. You want to hammer the heated spot TOWARD the dolly using light taps, then quench. The heat from welding DOES shrink the metal. I routinely weld fender patches in and use very little filler over the joint after hammering the oxy /acetelene weld over a dolly to "stretch" the metal and remove the distortion. I'm not going to get into all the technical reasons here because it is not necessary.
Your propane torch will work fine. Don't waste your money on the various shrinking hammers, they are not needed. You should know however, that MIG weld is much harder than an oxy/acetelene weld and will take a bit more work to smooth.
You should NOT have to weld washers to keep the area from "oil canning", this is what the shrinking is suppose to accomplish. Don't make cuts in the metal to relieve the distortion. Remember, you will still have to weld them closed and then it starts all over again. Believe me, I KNOW! You will possibly have to use some filler but, try to keep it to a minumum (1/16").
As I and others have said, try to find a book or someone that has done this type of work or try it on some scrap metal. Check out my website (themgdoc.com), this (body restoration)is what I do for a living. I do not currently show the shrinking process on the website but, you might see that I know what I'm talking about.
Good luck, John (The MGDoc)
Kemper, Sorry to say, the welding process does shrink the metal. When heating/welding the metal does expand. The shrinking takes place when the metal cools. Also, the "work hardening" takes place when working the metal cold. The heating and hammering really redistributes the molecules so the metal actually softens.
John
john

Semantics - I give up. Perhaps I should say: heat-expand cool-shrink, but it's not worth it.
Kemper

I have to say that I've used "Cold Fire" and it is nothing more than a hoax. It does prevent the discoloration caused by applying heat to metal, but that's about it. Everything else it does can be duplicated with a wet rag. Eastwood's heat-sync compound really does work though!

John, I know that you shouldn't have to weld washers on to prevent "oil canning", but its an easy fix for someone who's at their wit's end.
Jeff Schlemmer

It's more than just semantics; by shrinking and stretching we're describing fundamentally opposite things -- making the metal permanently "larger" or "smaller." (I'm not talking about expansion and contraction with temperature; those are temporary conditions.) IF welding did stretch the metal, the fix would be to shrink. I.e., if you had a low-crown panel and IF welding on it stretched it (a bit of a misnomer in the context, but by stretching I mean "makes the metal occupy a larger area at room temperature, once it has cooled"), then the crown would now be too high and the fix would be to shrink the area (i.e. make the metal occupy less area). Welding, however, SHRINKS the metal, so further shrinking by whatever technique will exacerbate the problem. The hypothetical low-crown panel will have less crown after welding, so it will need to stretched (hammered thinner so as to spread it out) to restore the crown.

Since David's cowl is a low-crown panel and is now lower than it should be, it means that the metal has shrunk and now has less crown that it should. It needs to be stretched to give it more crown and thus raise it.

Here's a perfect example:
http://www.metalmeet.com/forum/showthread.php?t=3807

Also see:
http://www.metalmeet.com/forum/showthread.php?t=1120

HTH!
Rob Edwards

Perhaps in your mind it's more than semantics, but that doesn't make it so. I'm not entirely sure why you're going to these lengths to make me "understand" and I assume accept your description of the process. I didn't ask the question that you keep trying to answer. So to end this nonsense, you're right, Rob. You win. That seems to be what you're looking for. Read the first three responses and then my first two. The suggestion was that the area (and as I stated, I wasn't familiar with the conversion process or the specific area) was stretched. The solution was to shrink the metal by heat and rapid cooling. My first direct response to you did stop short, I'll admit, but I purposely noted that heat expands the area. What happens after the heat isn't always NET shrinkage. I've seen MAE applications where shrinkage wasn't acceptable. The fill material will because it goes in molten and "occupying a larger area", as you say, but the base material won't unless you do something unusual (like excessive heat and/or rapid cooling, for example). No amount of unfollowed links that you post will make me want to change the context of what I said, especially since the horse was dead several posts ago. I've stuck a few pieces of metal together in my few days. I've seen the metal grow when the welder heat is high and sometimes I've seen it distort, warp, or shrink when the metal gets cold. I'm officially done with this one. I apologise to the rest of you that have had to weather this tangent.
Kemper

Kemper: No, I'm not trying to "win." It's not a competition. However, everyone is telling David to shrink the panel to fix his problem. I maintain that shrinking the panel is the opposite of what he needs and will make the problem worse, not better. He needs to stretch the panel to fix his problem. I'm just trying to keep him from digging himself into a deeper hole.

You may follow the links or not -- suit yourself. Makes no difference to me. I hope he does follow them, however.....

I appologise to everyone as well for this getting more heated (pun intended) than perhaps it should be! ;-)

Cheers!
Rob Edwards

I feel I should have described the panel a little better. It is only after reading the debate that you realise which facts about the situation are important.

The scuttle panel has a low crown in one direction. The weld line runs in that direction. I can see now that shrinkage along the weld line has pulled the crown down flattening it out along that line.

One of the links shows a bonnet someone has welded and they have placed a spirit level across to show the distortion. While the bonnet has a much higher crown than my panel the effect is similar.

Thank you everyone for your input.

David
David Witham

David, I stand corrected,after reading Rob's post and looking at the links. I guess I misunderstood your problem.
John
john

Well I have had a go. As I was working in the evening and panel beating is a bit noisy I tried to strech the weld with a bottle jack. I put a large bit of wood flat on the floor to spread the load, then a block on top to get the jack high enough. I pressed the underside of the weld line gradually working my way along.

There are a couple of places where I pressed too hard and will have to try some of the shrinking techniques detailed above. I worked from the edge of the distortion towards the middle and I am reasonably pleased with the result. Although when I follow some of those links that Rob posted and look at some of the pictures on John's MGDOC site I realise how much better it could be. However, the job is not finished as my bottle jack packed up before I had finished. It seems to have a pressure drain back problem!
David Witham

This thread was discussed between 02/03/2006 and 09/03/2006

MG MGB Technical index

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