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Here you see an original 12 bolt rear upper
control arm side by side with an original non 12 bolt arm. The only
difference is GM notched the driver's side (left side) arm to
provide clearance for the larger casing of a twelve bolt rear end.
A small patch was then spot welded onto the arm to restore the
strength in the arm around the bolt that was lost when the arm was
ground down to provide the needed clearance.
To get an exact copy of the patch that GM spot
welded onto the arm I used an old mechanic's trick. I used masking
tape to cover the patch. Tap around the outer edge with a hammer to
cut the tape into and exact copy of the patch
I then pealed the tape off and put it onto a
piece of poster board. This became my pattern for marking pieces of
cold rolled steel.
Here you can see the pattern sitting on a non
12 bolt rear upper arm side by side with an original GM 12 bolt rear
Here are two patches which were cut out using
a cut off wheel. I have since gone high tect. I took the pattern to
a shop and had 100 patches cut out with a laser. They are exact
The patch has been placed on the arm and the
arm bolt has been bolted into place to position the patch for
The patch has now been spot welded into place,
the arm has had the notch, needed for clearance on the 12-bolt rear
end casing, ground, and is ready to be powder coated.
Here is the original GM 12 bolt rear upper arm
side by side with the converted non 12 bolt rear upper upper arm
side by side (the black powder coated arm is the arm that was
This is a set of upper rear control arms that were
converted to 12 bolt arms and a set of
lower boxed arms that were done for