174 Section 2 Nonstructural Repairs
curved and, therefore, may not be a stable base. To
overcome this problem, a block of wood is placed on
the wheel house and the base of the jack to spread
out the force. At the damage correction end, a narrow
attachment is installed. The narrow attachment can be
used because the quarter panel overlaps the rear body
panel. Because the damaged area is made of two over- r r
lapping panels, it is stronger and less likely 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 addi-
tional 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 duckbill consists of
a stationary jaw and a movable jaw, Figure 8-20. 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 g
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 dimensions to the upset metal. The dolly
used for stretching must match the shape of the undam-
aged 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 8-21. If the center
of the damage is raised first, the crown will not be restored.
Wood block Hydraulic ram
Figure 8-19. This hydraulic ram rests on a block of wood
positioned between the ram and the wheel house.
Figure 8-20. The stationary jaw is located at the bottom
on this duckbill attachment. The movable jaw is at the top.
Normal contour
Damaged panel
Dolly
Figure 8-21. 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.