MATERIALS
DESCRIPTION PN SOURCE PRICE
Floorpan GMK-3020-505-64L Summit Racing $30.99
Weld-through coating Generic Local store $19.99
Seam sealer Generic Local store $9.99➔SOURCES
Miller Electric; MillerWelds.com
Summit Racing; 800/230-3030;
SummitRacing.com44 CAR CRAFT OCTOBER 2019
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which is much easier than attempting
a butt-weld seam, which is not neces-
sary for floorpan repairs.
Because Mustangs use a unibody
chassis, the floorpan is a critical
component. This means you have to
install the pan in the correct position
and replace all of the original spot
welds. The key areas for structural
rigidity are the front subframe and
crossmember braces under the
forward footwell. If you are replacing
the rear floorpans, the torque boxes
must be welded to the floorpan as well.
With the replacement pan in position,
the factory subframe components
were traced onto the new pan from
underneath, and we drilled^3 ⁄ 8 -inch
holes every couple of inches for spot
welds. You want at least as many
spot welds as the factory had; we
used more to ensure there were plenty
of solid weld points.
Before actually welding anything,
you need to prep all of the surfaces.
This means removing any rust or scale,
grinding to clean metal, then prepping
the clean surface with a high-zinc
weld-through coating. This special
primer protects the fresh welds with a
coating of zinc when the metal is
welded. Without this, the fresh welds
are subject to immediate rusting,
which you don’t want. After the weld-
ing is completed, we treated all of the
seams to a coating of seam sealer.
Replacing a single-side floorpan is
about a one- or two-day job, depending
on the level of experience and rust you
have to fight through. We made a few
patch panels with some scrap
16-gauge sheetmetal where the floor-
pan was rusted, but the new pan did
not cover. Floorpan replacement feels
like a big job, but it really isn’t. In fact, it
is quite forgiving of mistakes. Take your
time, be patient, triple-check your
placement, and weld ’er up!10. We used a stitch-weld process for the
perimeter of the pan, which is laying down
a series of 1-inch long beads a few inches
apart. Warping is not much of a factor on a
floorpan, but burning through the metal is,
so the stitch makes it easy and clean.10
11. Underneath the car, we used some
vice-grip clamps to squeeze the inner
floor edge to the rocker panel.11
12. Then we welded the top side inside the
car. You can see one of the small patches
we made to fill a small hole that the new
pan did not cover. If it was worse, we also
would have replaced the rear pan.12
13. After the welding was completed, the seams and spot welds were covered with seam
sealer to protect them and seal off any leaks.13
14. The last step was welding in the seat-mount pan. This is a similar process to the floor,
just align the seat-mount holes with the ones in the new floorpan and weld it in.14
9. The pan was repositioned in the car,
aligned, and the spot welds were made
first. This allows the pan to sit where it
needs to be, and you don’t have to worry
about any shifting during the rest of the
welding process.9