Rather than chopping and hacking at my firewall to upgrade the
heater motor in my CJ, I used a hole saw to cut the proper size
hole. This technique saved me from removing my battery tray, but I
did remove the passenger seat to make the work easier. Removing the
passenger seat is relatively painless. The rest of the procedure is
documented in Terry Howe's
Heater Motor Swap article.
Cut a 1"x5"x1/4" thick piece of aluminum. Aluminum is preferred
because it is soft and easy to work with. It will not grab on the
hole saw once the blade makes it through to the firewall
steel.
Mark the center of the bar and run it through with the hole saw
pilot bit.
According to measurements above, drill and tap the aluminum bar
for two #10 machine screws on the ends of the bar.
Align bar as per illustration. Mark hole locations on the
firewall and drill out for #10 screw clearance (~.195 bit
size).
Slip the bar through the existing blower motor hole and hold it
in place so that the two #10 screws can be inserted through the
firewall from the inside. Screwed them into the tapped holes in the
bar. Tighten the screws to pull the bar securely up against the
firewall on the engine side.
Use a 3 1/4" hole saw and run the pilot bit into the pilot hole
in the aluminum bar. Drilling from inside the Jeep, cut through the
firewall. I would recommend a variable speed drill for this and
never use the trigger lock on the power switch!! Hole saws tend to
bind up and smash elbows and fingers.
The result is a nice and neat hole in the firewall that seals
easily
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