184 Section 2 Nonstructural Repairs Copyright Goodheart-Willcox Co., Inc. to stretch than a single layer area. If the metal were a single layer, a broader tip would be used to prevent stretch. Once the body jack is in place, the pump is hooked up. Pumping the jack handle causes the ram is extend and push on the damage. A body jack can be used to push out lateral damage, such as damage to a body line. A piece of angle iron can be used to concentrate the push on the body line. When using a body jack, remember that the base and the attachment must be broad enough to prevent additional damage. The duckbill attachment can be used in confined areas to push out damage. For example, if a door is hit from the side, the duckbill attachment can be used inside the door to raise the damage. The duck- bill consists of a stationary jaw and a movable jaw, Figure 9-20. The stationary jaw is placed against a strong base. The movable jaw contacts the damage. The movable jaw must contact the damaged area squarely. If only a corner of the jaw contacts the area to be pushed, stretch damage may result. If the jaw cannot be positioned to contact the damage squarely, spread out the force with a block of wood. There will not be much travel in the movable jaw. It may be necessary to place wood blocks between the jaw and the damage to provide sufficient reach. Stretching When metal is upset, its surface area is reduced and its thickness is increased. Stretching restores the metal to its correct dimensions. To stretch metal, place a dolly on the inside surface of the damaged panel and use a stretching hammer on the outside surface. Hammering on the dolly causes the upset metal between the dolly and the hammer to thin out. This thinning is a rearrangement of the metal grains. As the metal gets thinner, its surface area is increased. The increase in surface area restores the proper dimen- sions to the upset metal. The dolly used for stretching must match the shape of the undamaged panel. Hold the dolly tightly against the upset metal. Hammer rapidly, with at least one blow per second. With each blow, the dolly should ring like a bell. Push the dolly out to meet each blow. When restoring the crown to an upset panel, stretch the panel starting from the ends and working toward the center. See Figure 9-21. If the center of the damage is raised first, the crown will not be restored. Tight double folds caused by longitudinal force will also need stretching. With the damaged panel under tension, the folds can be stretched to restore length. Do not hammer on the folds until the tension force Goodheart-Willcox Publisher Figure 9-19. This hydraulic ram rests on a block of wood positioned between the ram and the wheel house. Wood block Hydraulic ram Goodheart-Willcox Publisher Figure 9-20. The stationary jaw is located at the bottom on this duckbill attachment. The movable jaw is at the top. Goodheart-Willcox Publisher Figure 9-21. This diagram shows how to stretch metal to restore crown. Using the hammer-on-dolly technique, start at the edges and work toward the center. Normal contour Damaged panel Dolly