Heater Box Rebuild – GM A Body

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This article walks through a heater box rebuild for a GM A Body car. This write-up was based on our 1972 Cutlass Supreme “The Crusher Cutlass”, although the soft goods are very similar for its sister cars, such as the Chevelle. This box includes AC.

Here’s what I was starting with. The system worked, but leaked. Also since I had the fenders and wheel well liners off for the frame off restoration of the car, the timing made sense.






Shopping List:

I bought a rebuild seal set for the rebuild, however note that I was quite disappointing with the product. It was nothing more than cheap high density foam, and I could only use 3 of the seals out of the 10+ included.

Here’s what I bought, but I do not recommend it. CH17468 From OPGI.com.

Here’s what you’ll actually need:

  • 3m Strip Caulk – for the AC evaporator core to heater box seal.
  • Butyl rope seal (1/4″) – For the completed heater box to the firewall. I felt the rope seal was less messy than the wider strip caulk. although strip caulk could be used as well.
  • Foam or Neoprene Sheet/tile. With this you can cut your own blower motor seal (circular) and line seals.
    • Thicker sheets can be used for the seal around the AC lines entering the box. Thinner for the gasket around the blower motor. Look at your originals prior to ordering to get an idea.
    • A great source for foam that’s exactly like what comes in a kit is the shop foam floor tiles that you can buy at Lowes, Harbor Freight etc. They are large, cheap and you can cut any shape from them. And they are the right thickness. Amazon.com sell foam and neoprene sheets as well.
  • Chord/Rope seal for heater box halves. Butyl rope above could be used as well, but is messier and stickier, and you would want 1/16″ if you used butyl rope. Example: Neoprene Chord

Rope seal and some of the gasket kit:


Total cost, around 30-50 dollars depending on how much you buy of each of the above.



The bad news is that the hood, fender and wheel well liner on the passenger side will need to be removed to completely remove the heater box. If you are doing this process, chances are you probably already have them removed. If not, remove in this order: Hood, wheel well bolts, Fender bolts. You will need a second hand for the hood and fender to prevent dings. Also, save the exact location of each of the fender shims. This is critical for gap alignment.

Remove the box from the car. There are studs attached to the firewall which need the nuts removed. There are also smaller screws which thread into the firewall that need to be removed. You’ll also need to remove the heater core and AC lines. DO NOT allow water or contamination to enter the AC lines. Tape/Seal them off when disconnected. Remove all wiring. Don’t forget the ground and hot to the blower motor.

Once the entire heater box is removed, Break apart the halves by removing the screws around the perimeter. Remember which screws/bolts go where for reassembly. If you are missing bolts, or some ignorant prior owner used random parts from the bolt box, now is a good time to order the missing/correct bolts you need. My car was missing half of the necessary bolts, and a few others were cross threaded in which damaged the fiberglass box. Hardware stores have thread checkers that can give you the thread size of your good bolts.

Remove the AC evap core, which is probably stuck to the heater box from the factory strip caulk. Remove the air dam/metal plate. Remove the temp switch and resistor. Remove the blower motor.

Save all of your old gaskets for dimensions/use to compare to your new gaskets that you either ordered or will make.


Pictures of Disassembly





Clean the box inside and out with soap and water. Clean the evaporator core as well, however be VERY careful around the fins, and don’t let water inside. If you get water inside the core, you’ll need to dry it for days to ensure you don’t get water into your AC compressor, which will destroy it.

I also used this opportunity to remove the nasty, terrible, gross, ugly looking factory insulation around the thermistor. I basically use a razor blaze and CAREFULLY remove the material. Cut a line down the center and then chip off each half. The AC evaporator core is only aluminum so be careful not to damage it.

Once the box is apart and clean, locate or make your new foam gaskets first. Test fit each foam gasket for fit and seal prior to final assembly. Lay out all of your correct hardware as well. The butyl steps need to be LAST, because they are permanent.

Now is time to repair any stripped or damaged threads. Use epoxy to fill in any stripped threads. Let it harden, and then tap the threads with a self tapping screw. Make sure to drill the hole out to one size below the screw prior to trying to thread it, or you’ll just damage/crack the epoxy. DO NOT over torque. Inch pounds of torque are required for these bolts, not ft-lbs. Just over finger tight is required for everything threading into fiberglass.

After thread repair, I opted to paint the box as well while it was removed. Lightly sand the suface of the box with 200+ grit paper. I used primer and rattle can enamel. About 4 or 5 coats of paint were best to cover up the fibers of the fiberglass. Primer is recommended on fiberglass and helps fill imperfections. If the box is fully disassembled, there is no taping off required. Paint it all.

After Repair and Painting Pictures:






When ready, here’s the order of assembly if memory serves, although it’s fairly obvious.

  1. Fit the round line seals around the two AC lines. They go towards the inside of the box perimeter.
  2. Install 3M strip caulk/butyl rope around where the perimeter of the AC evaporator. Basically install it where you removed the old material. It goes behind the evaporator, and under the metal plate.
  3. Install the evaporator core and lightly stick to the strip caulk/butyl tape.
  4. Install the retaining plate. Make sure the little U section on the retaining plate lines up with the line from the evaporator core, which holds the core in place.
  5. Install the rope seal in the channel around the heater box halves.
  6. Press the two halves of the box together and tighten the bolts.
  7. Install the switch and resitor. No gasket is needed here.
  8. Install the blower motor cooler. No gasket is needed here. This is the small plastic square thing near the blower motor.
  9. Install the blower motor gasket and blower motor.  Note the blower motor does have a correct orientation. Look for the little guide hold to line up with the housing. Reference my installed pic below for correct orientation.
  10. Install blower motor Ground wire.
    1. This is CRITICAL! you will not be able to access this ground connection once the fender is back on. Make sure the ground has a good connection to the metal on the blower motor and will last a lifetime. For some odd reason this ground line is not shown on the assembly line electrical drawing, although trust me the motor will not work without it.
    2. Make sure the ground wire is long enough to go to where you will put it.
  11. Install butyl rope/strip caulk seal around the perimeter of the assembled heater box.
  12. Line up the heater box with the firewall studs and very lightly stick in place. Do not press hard yet, as you may need to remove the box to refit if the bolt threads don’t line up well enough.
  13. Install heater box bolts and nuts, pressing the box up/down/left/ride as needed to line up the bolts.
  14. Reinstall electrical connections and heater/AC lines.

Assembly Pictures:

There were some signs of heat damage on the bottom of the box near the header. With the upgrade to a stout big block and new larger tube headers going on, I opted to install adhesive heat shield on the bottom of the box. It can barely be seen when the box is installed, and should help the AC stay cool and keep the fiberglass/paint intact.

Completed Install before and after!

72 Cutlass Rocket 350 Engine Bay





Thanks for reading! Check out the whole project in our Projects Articles!