Welding; G-Model Underbody; Description; Underbody Alignment - GMC G Service Manual

2A-6 FRAME AND BUMPERS
13. Straight Crack
14. Sunburst Cracks
Figure 2 — Types of Frame Cracks

WELDING

Improper welding techniques are the cause of many
weld and/or frame failures. The following information points
out potential areas of difficulty and provides some general
guidelines for successful frame welding.
Most weld failures occur at the end of the weld in areas
of the frame that are under high stress. By eliminating the
ends of a weld, failures can be reduced; this can be done by

DESCRIPTION

The G-Model incorporates a unitized body design
comprised of side rails, cross sills, and outriggers which are
all part of the welded underbody.
A misaligned underbody can affect the operation of
many vehicle components. It is essential that underbody
alignment be exact to within 1.6mm (1/16-inch).

UNDERBODY ALIGNMENT

To determine the alignment of the underbody, it is nec­
essary to use a good quality tramming gage. This gage
must be capable of measuring all points of the underbody
up to three quarters of the body length.
B-09088

G-MODEL UNDERBODY

making a hole or slot in the part to be attached and then
using a fillet weld around a slot or plug weld. Do not leave a
notch at the end of a weld.
Over-welding can be harmful, especially if it is in an area
of the frame that receives high concentrations of stress.
Small cracks in a crossmember may be welded. (Refer
to "Repairing Cracks" previously outlined in this section.)
When welding:
1. Do not use oxyacetylene welding equipment.
2. Whenever possible, use smaller diameter electrodes
and make several passes; this is preferred to using a
large diameter electrode and making fewer passes.
3. Do not use more heat than is necessary to give good
penetration.
4. Do not run more passes than necessary.
5. Make sure the weld is free from craters and
undercuts.
6. Make sure scale is removed from each successive
pass.
7. If repairing a crack, grind the surface of the weld flush
with the parent material.
8. When mounting accessories, do not weld across the
flanges.
9. When mounting accessories, do not weld within
19mm (3/4-inch) of a flange.
10. Do not weld up to the edge of a part being welded to a
frame. Leave 6mm (0.25-inch) between the end of the
weld and the edge of the part.
11. Do not weld cast brackets to the frame.
12. Do not weld the flanges of cracked reinforcements
and base rails together.
13. Connect the welding machine ground cables as close
to the working area as possible.
14. Avoid direct contact between the welding cables and
any part of the vehicle.
15. Do not get the welding cables near the vehicle wiring.
Following the instructions given by the manufacturer of
your gage, measure the horizontal and vertical points as
shown in figure 3. Compare each measurement with the
specifications given in the figure to determine the damaged
areas and the extent of the damage. When repairing the
underbody it will be necessary to return the underbody to
these original specifications within 1.6mm (1/16-inch).
Many different types and brands of unibody repair equip­
ment are available; each system may be slightly different. A
multiple pull system is most desirable in order to correct the
damage in the same direction in which it was created.
When making multiple pulls, use the last in, first out proce­
dure. This means that you correct the damage in the
reverse sequence by which it was created. It is also impor­
tant to correct the damage in the opposite direction in which
the damage was created.