Checking Frame A Lig N M E N - GMC 1986 P-Truck Service Manual

2A-4 FRAME
• Do not cut holes with a torch.
• Do not overheat the frame rails.
• Avoid welding on the flanges.
• Do not allow four or more holes to exist on
the same vertical line of the web.
• Holes made in a reinforcement should be
placed a distance of at least two times the
material thickness from the edge of the
reinforcement.
CHECKING FRAME ALIGNMENT
The main parts of a frame are the side rails and
crossmembers. The rails carry the load and the
crossmembers stabilize the rails.
Types of frame misalignment can be divided into five
groups. The five groups are sag, buckle, diamond,
sidesways, and twist. For a brief definition of these
terms, refer to " Description" at the beginning of this
section.
A misaligned frame rail may have moved forward
from, up or down from, or to the side of where it should
be. These possibilities must be checked.
The easiest way to check frame alignment is with
gages made for this purpose. Detailed instructions are
normally supplied with gages at the time of purchase.
Therefore, instructions for gage use are not given in
this manual.
Figure 1—C/K And P Model Frames
It is possible to check certain portions of frame
alignment without the proper gages. The procedure is
described later in this section.
Whether alignment is checked with or without gages,
the vehicle must be parked on a level section of floor.
Certain conditions call for preliminary checks before
actually checking the frame. Suspension or axle
problems may make it appear that the vehicle frame is
out of alignment. If an axle has shifted, " Diamond" or
" Sidesway" may appear to exist when, in fact, they do
not. A weak spring may make the vehicle appear to
have a twisted frame.
A visual inspection of the top and bottom flanges of
each rail may reveal the specific area where sag or
buckle exists. In the case of sag, wrinkles may appear
on the top of the upper flange; such wrinkles are
definite evidence of sag. Wrinkles on the bottom of the
lower flange are definite evidence of buckle
DETERMINING THE FRAME CENTERLINE (Figure 1)
1. Move the vehicle to a level, well-lighted section of
2. Make a diagram of the frame on the floor beneath
the floor.
the vehicle as follows:
a. Using a plumb bob, transfer points 1, 2, 11,
12 from the inside of the web to the floor.
Mark the points (figure 1).