Straightening Fram; Repairing C Ra C K - GMC 1986 P-Truck Service Manual

b. Using a plumb bob, transfer the remaining
points from the outside of the web to the
floor. Mark the points. The actual placement
of these points is not so important as is the
fact that the points must be taken from the
same place on both the right and left frame
rail.
3. Move the vehicle away from the points on the
floor.
4. Measure the distance between points 1 and 2.
This distance should be 704.85 mm (27.75-inch)
for the standard front axle, and 847.25 mm
(33.75-inch) if equipped with a 5000# I-Beam
Front Axle.
5. Measure the distance between points 11 and 12.
This distance should be 847.25 mm (33.75-inch).
If both the measurements in steps 4 and 5 were
c o rre c t, c o n tin u e the
m e a su re m e n ts are not c o rre c t, re fe r to
" H orizontal C h e ck" step 2, and mark the
centerline through the intersection of two sets of
diagonals that have equal measurements.
6. Make a mark on the floor exactly halfway
between points 1 and 2. Make a similar mark
between points 11 and 12.
7. Use a chalk line to mark a line through the points.
This is the centerline of the frame.
HORIZONTAL CHECK
1. Measure the distance between the frame rails,
and the centerline at points 3-10. Each pair of
co rre sp o n d in g points should be an equal
distance from the ce nte rlin e w ithin 3 mm
('/s-inch ). Example: Points 3 and 4 should
measure equally from the centerline as should
points 5 and 6, 7 and 8, and points 9 and 10.
2. Measure diagonally from point 1 to point 6, and
from point 2 to point 5. Mark the floor where the
diagonals pass the centerline. The diagonals
should measure the same distance within 5 mm
(3/ie-inch), and should cross at the centerline. If
the frame is within specifications, the frame
within these points is properly aligned.
3. Repeat step 2 on other pairs of diagonals until
the entire frame has been checked. Example:
Point 3 to point 10 and point 4 to point 9 or point 5
to point 12 and point 6 to point 11, etc.
4. Refer to " Straightening Frames" to repair any
damage that is found.
STRAIGHTENING FRAMES
Frame straightening can be complicated and usually
requires special equipment. It should be attempted by
competent personnel only.
A complete analysis of the condition of the frame
should be made before any steps are taken to
straighten it. Analyzing the cause of failure will help
p ro ce d u re . If the
determine the correct sequence of steps in the repair.
(See " Possible Causes" under " Diagnosis" and
" Minimizing Frame Service" previously outlined in this
section.) Corrective procedures should be set up to
reverse the flow of the damaging forces. In many
instances, to reverse the flow of forces, pressure must
be applied from different directions at the same time.
Careful use of controlled heat is important. Too much
or too little heat, or the improper application of heat, is
a major source of trouble. Frame heating should be
done with a large sized tip (multi-hole heating tip) and
a neutral flame should be used (oxygen adjust opened
just enough to remove the feathers or stringers from
the blue center at tip of torch). Heat the area that will
be stretched as frame is straightened. Heat the area
AFTER S U F F IC IE N T PRESSUR E HAS BEEN
EXERTED to cause a slight checking, or silvery cracks,
to appear on the surface of the metal. Gradually
increase the pressure while heating. Do not heat
beyond 606°C (1200°F). A red glow indicates that the
m aterial is overheated. O verheating w ill cause
distortion and stretching, as well as a change in the
characteristics of the metal.
REPAIRING CRACKS
Two common types of cracks are shown in figure 2.
The straight crack will normally start from the edge of a
flange. It will go across the flange and through the web
section of a rail. Finally, it will continue through the
other flange. This type of crack may result from high
concentrations of stress in small areas of the frame,
excessive bending moment, and torsional loading.
(See " Minimizing Frame Service," previously outlined
in this section).
Sunburst crack(s) will radiate out from a hole in the
web section of a rail or crossmember. They are caused
by high loads being applied at a mounting bracket or
crossm em ber which is not securely or properly
attached to the rail.
If cracks occur to both the rail and reinforcement at a
particular area of the frame, they must be repaired
separately. The flanges must react independently to
prevent localized stress concentration. Use a copper
spacer between the flanges of cracked base rail
flanges and reinforcement flanges.
Crossmember mounting flange cracks may be
repaired in the same manner as side rail cracks.
However, weld bead should be built up to provide a
good smooth radius. If a crossmember is greatly
damaged it should be replaced.
It may be necessary to align the frame and level the
rails before repairing the frame.
1. Remove any equipment that will interfere with
access to the crack.
2. Locate the extreme end of the crack and drill a 6
mm (0.25-inch) hole.
FRAME 2A-5