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Q. I was trying to weld the center hood trim to the top of my '49 Chevy hood, and I warped the metal. I was going to make relief cuts in the metal, then bolt two pieces of flat bar metal to both sides, and re-weld the metal to straighten it up. Is there a better way to go about straightening the hood?
Grove, Oklahoma, Via the Internet
A. First, we need to understand what caused the problem initially. Whenever you weld, the filler metal is put down red hot, and as the weld and the hot metal next to it cool to room temperature, they shrink, pulling waves into the metal next to the joint. Every bit of the warping you have is caused by shrinking in the weld area and the heat-affected zone next to the weld. If you make relief cuts in the metal, and weld these joints back together, that will cause more shrinking, and you'll have even more distortion. Bolting flat bars to the hood won't help much at all.
Unfortunately, your options for fixing this are pretty limited. The early Chevy hoods were made in two halves, with a flange where they join in the center. If you cut the flange off, then the joint can be butt-welded, and if the weld is made with an Oxy-Acetylene torch or with a TIG welder, the weld can be stretched by hammering on-dolly directly on the weld bead. If you stretch the metal by hammering exactly as much as the welding shrank it, the distortion will be completely eliminated. While this may sound easy, it's actually quite a difficult task, and even a very experienced metalworker would find it challenging, and would probably go back and forth between hammering and heat shrinking a bit before it was finished. The front of the hood is easy; it's the nearly flat area in the middle and rear of the hood that's difficult.
If you didn't cut off the flange, and/or if you MIG-welded the seam, you simply can't hammer effectively on the weld. I'm afraid this is probably the situation you are in. The best fix is to cut a strip about 2 inches wide from the center of the hood, removing the weld, and butt welding a strip of new metal into place, using TIG or gas welding. A gas weld is very soft, but it will cause an alarming amount of distortion, since the area heated by the welding flame is quite large. That's why I prefer TIG welding for these difficult areas-it causes much less distortion, and it leaves a bead that can be easily stretched by hammering.
Some people have gotten away with joining hood halves by MIG-welding the joint, but doing everything possible to keep the heat to an absolute minimum. This requires tack-welding the entire joint, starting with tacks spaced about a foot apart. Then place new tacks halfway between the first tacks, divide those spaces in half with new tacks, and continue until the tacks are no more than an inch apart. It's essential to let the metal cool completely wherever you are placing a new tack weld. Then, you can start adding a tiny bit of weld to a tack, move about 6 inches away, and add a tiny bit of weld to the tack there, and continue in this way from one end of the hood to the other. This way, you have the lowest possible amount of distortion. Note that I didn't say you won't have any distortion. I'm not recommending this approach-just reporting that some people have "pulled it off" this way with a MIG welder. Of course, you can expect to spend several hours welding the entire seam, with all the time needed for cooling, and it's likely some hammering will be required, too.
I was at Fatman Fabrications recently, and Brent VanDervort, the owner, was joining the center seam of a hood like yours by soldering the flanges together. To do this, you have to disassemble the hood, clean the flange edges meticulously, tin them, clamp the hood halves together, and solder the joint. It worked great for Brent. Many people question whether a soldered seam like this will have adequate strength-remember that most automotive gas tanks are soldered together, and they form a liquid-tight joint that usually lasts longer than the car!