October 2012 Professor Hammer’s Metalworking Tips

Q. I’m struggling to repair the rolled outer fender lip on my ’40 Chevy truck, which is shaped like the letter “J.” I’ve tried using a hammer and homemade dolly, but I end up with an inconsistent shape. I thought of making a new strip with the lip already formed, and then weld it onto the fender. Would that work?

Larry Mullen 
Via the Internet

A.Oftentimes, cutting off the bad stuff and starting with good, clean, full-thickness metal is the best approach. The tool of choice for putting the proper contour in the repair strips for your fender edges is a metal shrinking and stretching machine. Within their limits, they can do a great job of making straight profiles curve any way you want.

First, find the exact radius on the edge of your fender, which is probably close to 5⁄8 inch. An easy way to measure this is to test-fit small pieces of round steel-bar stock inside the fender edge to see what size fits snugly. Once you know this dimension, you can start making the new fender edges.

Cut strips of metal to a convenient size and bend a flange on one edge. This flange will be bent against a round bar that will give it the radius of your fender edge. Let’s say it’s 5⁄8-inch diameter for this example, although your truck may be slightly different. If you have a sheetmetal brake, you can make a simple radius die that will give you this profile.

If you don’t have a brake, you can tack-weld a piece of 5⁄8-inch (or whatever) round bar to a piece of larger material, let’s say 1-inch square tubing. Then you can clamp this bending fixture on top of a piece of sheetmetal on the edge of a sturdy workbench, and bend up a 90-degree flange by hand.

If hand-bending, you’ll probably need at least 6 inches of metal sticking out to get enough leverage to make this bend, and you may not be able to make a good bend in pieces much wider than 2 feet. The advantage of using a bending brake is that you can make accurate-length flanges that won’t require trimming after bending, and you can easily make your pieces 3 or 4 feet long. I’d suggest starting with an angle having a 1-inch flange on one side, and a ¾-inch flange on the other.

With all your pieces formed, you can use your stretcher on the longer flange to start shaping each piece to match the contour of your fender. You will have excellent control of this process, and if you go too far with the stretcher, you can make corrections with the shrinker until you have the piece shaped perfectly. You’ll probably have to do a fair amount of shrinking/stretching on both flanges to get the shapes dialed in to match the contours of your fender in both planes.

Once each piece is shaped to your liking, you can clamp it into place so it overlaps the fender, and scribe the edge for trimming. Now, you can cut away the old fender, and butt-weld the new piece into place.

Once all the pieces are welded into place, the last step is to curl the inner flange up, going from a 90-degree “L” shape to the “J” shape your fenders originally had. It’s best to make a special dolly for this, and it’s easy to do by welding a short piece of round steel bar to a hefty chunk of solid steel. This “radius dolly” should weigh at least a couple pounds to be effective. Now you can go all around your fender edges, holding your radius dolly tightly against the fender edge, and coax the inner flange into the “J” shape of the original.

While this involves many steps, you’ll find each step is very controllable, and I think you’ll be pleased with the result! My DVD, “Scratch-Building a Fender” shows this process in detail. CT