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I have been looking to find details on the type of seal used on the heater core where it passes through the firewall. I’ve read the post on the core replacement, but it mentions reusing the seal. Mine is old and torn. The only suggestion I have is to go to a hardware store and pick up some hvac foam. Any help is appreciated.
It's not F-body related, but I've used a product for this same purpose on some of my old dinosaur cars that needed heater cores (where the original seals were no longer available). It's called rope caulk, and it's an excellent formable, non-hardening caulk that can be built up and shaped into whatever space you need to fill and seal. It will resist all types of weather and last, well, indefinitely (at least it has so far in my applications):
And here is what the product looks like as you unroll it (again, you can build it up and form/cut/rip as needed; it's very workable and will stick to itself to form whatever shape you want/need):
Other thing I've used but have access to at work you can get it too is refrigeration line armaflex insulation.
Go to a local HVAC supply house and buy a piece of 7/8 ID 1/2 wall armaflex tubing. This is the black insulation used on home ac suction lines.
Then take a section of it split it down the middle to filet it open and you will have a flat sheet to work with then cut it to size of heater fittings and usually about a 5x5 4x5 section slide it over heater pipes and then slide the case back it. It's the same material as it had once upon a time.
Other thing I've used but have access to at work you can get it too is refrigeration line armaflex insulation.
Go to a local HVAC supply house and buy a piece of 7/8 ID 1/2 wall armaflex tubing. This is the black insulation used on home ac suction lines.
Then take a section of it split it down the middle to filet it open and you will have a flat sheet to work with then cut it to size of heater fittings and usually about a 5x5 4x5 section slide it over heater pipes and then slide the case back it. It's the same material as it had once upon a time.
This is another good suggestion. Not quite as readily available as rope caulk, but it's another great option.
I'm going to have to remember rope caulk myself for the times when the heater cores not removed. Even on varying vehicles not just F bodies.
Believe it or not you can get a lot of underhood air forced into the HVAC box through there and also loose air flow from inside the vehicle. Those seals are very important and overlooked a whole bunch
Sorry for replying later than I wanted. Between cold weather and job, I haven’t had a chance to get back out there. The rope caulk did a great job since it was flexible. The firewall hole is about 1.5”x3” so I needed quite a bit to fill the space. Once I get the car running (who knows when) I’ll need to report back and let you know how it held up. Thanks again for the assistance!
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94 Heater Core Firewall Seal 
