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Old 07-30-2005, 10:42 AM
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Welding stress fracture in sheet metal

I have a 40 year old fender that has a small stress crack (3/4" long) on the lip that forms the inner edge for the hood opening. The crack is located where the fender is supported by the grill surround.

I would normally open up the crack and weld or braze a small piece of metal behind it to form a doubler to give added support to that area. But, due to its location I cannot increase the thickness of the metal.

What are my options for repair? First I thought that I should drill a small hole where the crack ends to keep it from traveling further.

I could MIG weld the crack, but I fear the concentrated heat will embrittle the metal next to the weld causing it to crack next to the repair.

I could Oxyacetylene weld the crack, thinking that the additional heat may anneal the area if cooled slowly thereby making the repair less likely to crack, but not sure.

I don't think I should braze the crack since the metal does not overlap; I am afraid the repair would not hold up.

Any thoughts? Thanks for your help!

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Old 07-30-2005, 10:57 AM
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A highly stressed area like that is not a good place to MIG since the MIG bead is so hard and relatively brittle. Brazing is a poor substitute, again because it is weaker than the surrounding metal and will eventually break in exactly the same place. You see a lot of old fenders with cracks that were repaired like that and are re-broken. IMHO, oxy/acetylene is the only proper fix of those fender edge tears. No need to pre-drill the crack root. Use a clamp to hold the edges together and run your welding bead right down the center of the crack using standard 1/16" mild steel filler rod. A little grinding and the fix is as strong as the base metal. Although gas welding dumps a lot of heat into the metal compared to MIG or better yet TIG, edges of fenders are almost always in a highly compound curved area which are much more resistant to heat damage than flat body panels. There may be a slight shrinkage dip next to the weld that you can easily tap out but there won't be that dreaded full panel warpage.
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Old 07-30-2005, 12:13 PM
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it's woth getting tig welded. cracks are a tuff fix at best.
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Old 07-30-2005, 08:39 PM
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A small 3/4" crack shouldn't be anything to get all concerned about IMO, I'd gas weld it without hesitation, or a mig should work fine-just put a good weld down that's hot enough so it isn't brittle, don't over grind the weld causing the basemetal to thin and it should be fine. Or you could section 1/4" out of the cracked area and make a filler out of 1/4" round stock welded in place then grind to shape. I agree a small backing properly brazed in place would add a lot of strength and would be more crack resistant than just welding the crack. 40 year old fender, is this car a 55?
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Old 07-31-2005, 08:28 AM
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Thanks everyone. I thought oxyacetylene may be a better choice here, but I was not sure. The crack is small and it is not structurally critical but I want to repair it for cosmetic reasons, even though you can only see it with the hood open.

The car was built in October 1965; registered as a '66 when it was first sold. It is an MG Midget. Not exactly a hotrod, but you know those yappy little mutts that think they're big dogs? Same thing here.
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Old 07-31-2005, 09:00 AM
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I guess I need math help,
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Old 07-31-2005, 09:45 AM
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The only problem with MIG is using the wrong wire for the job. I would weld that with .023 with an ER70S2 or 3 wire. Not always available since most welding is structural and most suppliers only stock 'S6 or 7. The S2 is made for doing sheetmetal. It grinds easy and has less shear strength than the others. Perfect for body panels. Zap it in 1 spot at a time overlapped so it looks like a tig weld and forget it for life. It works for me.
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