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I have managed to make it through the disassembly process in order to access a leaking heater core on a 68 with AC. The heater box is the only item remaining. I have removed four hex head bolts that go into the firewall but the box will not budge. It appears to have some sort of adhesive between the flange and firewall. My assumtion is that the box will come loose after cutting this adhesive away. Am i on the right path or are there additional fasteners that i'm not aware of?
I just did this. There are a number of things to check...
First, there should be three small screws on the left of the box (that hold it to the ducting. Second, there are three bolts accessible from the inside that must be removed. Third, there are three nuts and one bolt on the engine side. They are two nuts by the heater blower, one on the upper driver's side of the box, and a bolt down underneath. Just search with a flashlight on both sides. Then you just ever so gently break the seal and pull the A/C box forward. Then you can slide the heater box out from under the dash.
1. Disconnect the heater core hoses from under the car.
2. Because of the geometery of the copper core pipes, the box will come out only one way, bringing out an inch, then tilting it face down
My box was stuck fast to the interior rubber firewall sound mat, and it took a good knock to break free. There was no adhesive, just 30 years of normal operation.
On my '71 I had the same problem. It did feel like it was glued to the firewall. After a few bad words and some pulling it came loose but it still did not come out. I had to lift up and tilt out wards to get the box off.