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UNDERBODY
3-7
ALIGNMENT CHECKING PROCEDURE
The underbody is comprised of body dash front
braces, body floor pan cross braces, body com-
partment pan side rails, inner and outer rocker
panels and other floor panel components. The
underbody is of all welded construction. Misalign-
ment in the underbody can affect front fender, door,
rear compartment lid and glass opening alignment,
station wagon tail gate and back body opening
alignments. Most important, however, underbody
misalignment can influence the suspension system,
thereby causing many of the problems that arise
from a suspension misalignment. It is essential,
therefore, that underbody alignment be exact to
within 1/16" of the specified dimensions.
In the event of collision damage, it is important
that underbody alignment be thoroughly checked
and, if necessary, realigned in order to accurately
establish proper dimensions.
There are many
classifications of tools that may be employed to
correct the average collision damage situation
including frame straightening machines, lighter
external pulling equipment and standard body jacks.
NOTE:
Minor misalignment of the front end
assembly to the body may be corrected by adding
or removing shims at the upper skirt-to-dash
attaching surface. Figure 3-6 shows both upper
and lower attachment and the installed position
of the shims. Shims are available in 1/32" and
1/8" thickness.
CAUTION: Do not change skirt assembly-to-
dash shimming in an effort
to adjust the
door-to-fender gap or any other sheet metal
appearance item. These shims regulate the front
end assembly relationship to the body and should
only be used to correct dimensions as shown in
Figures 3-7 and 3-8.
To assist in checking alignment of the underbody
components, repairing minor underbody damage or
locating replacement parts, the following underbody
dimensions and alignment checking information is
presented.
Body Tram Gage
An accurate method of determining the alignment
of the underbody utilizes a measuring tram gage.
The tram gage required to perform all recom-
mended measuring checks properly must be capable
of extending to a length of 91". At least one of the
vertical pointers must be capable of a maximum
reach of 17".
Horizontal dimensions shown in the upper portion
of Figures 3-7 and 3-8 are calculated on a plane
parallel to the plane of the underbody. Precision
measurements can be made only if the tram gage
is properly adjusted so as to remain parallel
to the plane of the underbody during measuring
operations.
A proper tramming tool is essential for analyzing
and determining the extent of collision misalign-
ment present in underbody construction.
Principles of Tramming
In the upper portion of 3-7 and 3-8 all reference
locations shown are symmetrical about the center-
line of the vehicle. For example, when performing
a cross-check of the body floor pan area, dimen-
sion "N
; ;
should measure the same distance in both
diagonal directions of the cross-check operation.
Cross checking operations are used to determine
the relationship between two locations on the
underbody.
To measure the distance accurately between any
two reference points on the underbody, two specifi-
cations are required:
a. The horizontal dimension between the two
points to be trammed.
b. The vertical dimension from the datum line to
the points to be trammed. As an example, the
diagonal measurement (calculated on a hori-
zontal plane) between reference points of di-
mension line "N", shown in Figure 3-8, is
78 25/32 inches. The specifications from the
datum line have a vertical height difference of
3 5/16 inches between the forward location of
dimension "N" (at vertical dimension "e")
and the rearward location of dimension "N
;;
(at vertical dimension "h").
The vertical
pointer used at the forward location should
be positioned so as to extend 3 5/16 inches
further from the tram bar than the pointer
used at the rearward location. With the proper
settings the tram bar will be on a plane
parallel to that of the body plane. The excep-
tion 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 gage should be set at
the specified dimension to check the accuracy
of the repair operation.
Car Preparation
Preparing the car for the underbody alignment
check involves the following:
1. Place the car on level surface.
2. The weight of the car should be supported at
the wheel locations.
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