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Fender bumping and repairing is not as difficult a job as the damaged metal might indicate. A good job is largely a matter of studying the bump or jam and selecting the proper tools to work the bump out. The operator must always keep in mind the fact that the jammed section must be taken out the same way it went in.

The reason for this is that when a fender is damaged, a backward strain—or strain contrary to that set up in the metal when the fender was formed—is set up. This strain must be relieved before the metal can be made to assume its original shape.

To clearly illustrate this job we take a fender that has actually been damaged and follow the steps in its repair, one by one. The fender is a rear fender that was jammed by being struck in the front and, in passing, the car that did the damage pulled out the rear of the fender. The crown is jammed in and the blow on the rear of the fender caused the outside flange to stretch.

First clean the fender thoroughly with an oil-soaked rag. The purpose of the oil is twofold: first, to reflect the light onto the damaged section, so that every bump and indentation may be readily seen; second, the oil creates a cushion under the hammer and keeps the paint from being picked up by the hammer.

Then clean the underside of the fender of all grit, tar, and clinging dirt particles. If this is not done, these particles will be beaten into the metal when the hammer and dolly block are used.

Rough out the jam, using the proper dolly block. Follow the crown of the fender when roughing out this jam. Never use a ball peen hammer to rough out a jammed fender or the metal will be full of small "bubbles" and a smooth job will be an impossibility. Selection of the proper dolly block will greatly simplify this operation, because the right block has the same contour as the fender had before it was jammed.

The jam at the rear of the fender shows that the rear skirt has been pulled out and also that the rear corner section is pulled out, making a series of short tight jams. The best method of roughing out short, tight jams is to use two dolly blocks, one held on the bead of the fender to relieve vibrations and the other to rough out the bump.

After roughing out these tight jams, bring the skirt back into alignment by holding a dolly block behind the low spot and the skirt and hammering the skirt back.

Hot metal shrinkage will draw the skirt of the fender in and make it rigid. Then, in the centre of the stretched part of the crown, at the highest point of the bulge and about 2 inches up from the bead apply hot shrinkage.

Caution: The pressure must be applied to the rear of the fender throughout the entire hot shrinkage process.

If there are any breaks or cracks in the fender they must be welded before the hot shrinkage operation.

Bump out the shrunk section, using a dolly block and a bumping hammer or mallet. Hold the dolly block loosely on the low spots underneath the fender and strike the high spots on the outside with the hammer or mallet. This method causes the dolly to spring back against the low spots and readily smooth them out.

If the fender does not set square or is not shrunk enough, apply more pressure and repeat the process. Always heat on the highest part of the bulge.

To bring the bead into perfect alignment, hold a dolly block behind the low spots and strike the high spots on the bead with a mallet or bumping hammer.

To show up the high and low spots on the rough bumped sections, use a long piece of chalk and holding it flat on the metal, rub the chalk on these sections. The high spots will show up as being white with chalk while the low spots will be dark or unchalked. Do not file yet.

Finish bumping the sections by holding the dolly block loosely on the low spots and striking the high spots with a bumping hammer. Check the smoothness of the metal by rechalking ; if a smooth job has been done the metal will show up as thoroughly chalked.

The metal may now be finished and prepared for painting with a file and disk sander. At the time the file is being used, any small indentations will show up and may be easily tapped out.

Front fender bead cracked

Most front fender damage is caused by a front end collision, or a blow directly on the front of the fender. It is not enough to simply repair the damage done to the fender, as the fender bracket was pushed back out of alignment at the same time the fender was damaged. With this condition, all of the strain and weight of the fender is thrown onto the outside flange, or bead, causing the metal to crack.

To relieve this strain, remove the bumper and place a 2 X 4 on the tire and under the end of the fender and pull up on the end of the 2 X 4 until the bead of the fender buckles into its normal position. Then block the 2 X 4 in this position. Heat to a cherry red a spot about 2 inches long on the back flange of the fender bracket at a point approximately 14 inches from the outside edge of the fender.

Also heat a spot about 34 inch long on the forward flange of the bracket.

With the pressure still applied, allow the bracket to cool. This will normalize the bracket, causing the strain to be relieved and preventing the fender from cracking at the bead.

When it becomes necessary to repair cracks that have been caused by failure to relieve the strain in the bracket, it is necessary to weld the break before the bracket is normalized.