Chevrolet Light Duty Truck 1973 Service Manual page 170

2-2 FRAME
2. Dimensions must be within 3/16".
3. If a tram bar is used, for horizontal alignment "X "
- check from opposite and alternate reference
points AA, BB and CC, as illustrated by the lines in
Figure 2. Error will result if a tram bar is not level
and centered at the reference points.
4. Obtain vertical dimensions and compare the
differences between these dimensions with the
dimensions as shown in chart.
Horizontal Check
1. Measure frame width at front and rear. If widths
correspond to specifications, draw centerline full
length of vehicle halfway between lines indicating
front and rear widths. If frame widths are not
correct, layout centerline as shown in Step 4.
2. Measure distance from centerline to corresponding
points on each side of frame layout over entire
length. Opposite side measurement should corre­
spond within 3/16".
3. Measure diagonals marked A, B and C. If the
lengths of intersecting diagonals are equal and
these diagonals intersect the centerline, frame area
included between these points of measurement may
be considered in alignment.
4. If front or rear end of frame is damaged and width
is no longer within limits, frame centerline may be
drawn through the intersection of any two
previously drawn pairs of equal, intersecting
diagonals.
Vertical Check
Vertical dimensions are checked with a tramming bar
from indicated points on the frame (figs. 3 and 4). For
example, if the tram bar is set at point B with a vertical
pointer length of 8-1/4 inches, and at point E with a
vertical pointer length of 5-1/4 inches (a height
difference of 3 inches), the tram bar should be parallel
with the frame. If the area is twisted or misaligned in
any way, tram bar will not be parallel. Placing the tram
bar vertical pointers on opposite sides of the frame side
rail is preferable in that frame twist will show up during
this vertical check. Figures 3 and 4 show typical checking
points, with dimensions for various frames shown in the
chart below.
Frame Repair
Welding
Before welding up a crack in frame, a hole should be
k
drilled at the starting point of the crack to prevent
spreading. Widen V groove crack to allow complete weld
penetration.
NOTE: Do not weld into corners of frame or
along edges of side rail flanges. Welding at
these points will tend to weaken the frame
and encourage new cracks.
Bolting
Wherever rivets or failed bolts are replaced, bolt hole
must be as near the O.D. of the bolt as possible to
prevent bolt from working and wearing. Drill out and
line ream hole (or holes) to the bolt O.D.
UNDERBODY ALIGNMENT
One method of determining the alignment of the
underbody is with a tram gauge which should be
sufficiently felxible to obtain all necessary measurements
up to three quarters the length of the vehicle. A good
tramming tool is essential for analyzing and determining
the extent of collision m isalignm ent present in
underbody construction.
MEASURING (Fig. 5)
To measure the distance accurately between any two
reference points on the underbody, two specifications are
required.
1. The horizontal dimension between the two points to
be trammed.
2. The vertical dimension from the datum line to the
points to be trammed.
The tram bar should be on a parallel to that of the body
plane. The exception to this would be when one of the
reference locations is included in the misaligned area;
then the parallel plane between the body and the tram
bar may not prevail. After completion of the repairs, the
tram gauge should be set at the specified dimension to
check the accuracy of the repair operation.
EXCESSIVE BODY DAMAGE
If damage is so extensive that key locations are not
suitable as reference points, repair operations should
always begin with the underbody area. All other
components should be aligned progressively from this
area. Unlike the conventional type of frame design, the
unitized type of body construction seldom develops the
two conditions of "twist" and "diam ond" in the
underbody area as a result of front or rear end collisions,
therefore, there usually is an undam aged area suitable as
a beginning reference point.
LIGHT DUTY TRUCK SERVICE MANUAL