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Would anyone have a suggestion for a replacement gasket for the heater case to firewall space? The case came off and left the original in mostly one piece but it is dried out and not really a gasket or seal anymore. I could source the new heater core but not a new seal.

Are you talking about the heater plenum where it screws to the firewall under the hood?

Yes, its the thick gasket that goes in between the heater case and firewall.

I was not aware that a gasket was used there. I thought they just used that RTV type stuff.

I have to replace my heater core and I intend to use black RTV only to seal it up.

Well, There is a problem with just using a liquid solution. The surface of the heater case that goes against the firewall cannot fit flush against the firewall. The two cases that fit together in the engine compartment side of the firewall (A/C evaporator, and Heater Core) are molded to fit to each other with a tight seam, but are not able to fit to the firewall close enough to just "glue" them on. Moreover, if you were to try to modify one to fit flush, you would have to do the same to the other. The case has a series of stand offs, about 3/8" molded all around the surface where it touches the firewall. I suppose you could pull the entire suitcase assembly off and grind it flat, but there might then be other clearance issues.

I used rope caulk on mine when I rebuilt it. It’s about 1/4” round. Looks like rope. Sticky tar like substance. You can get it in different sizes. Just cut off what you need. Works great. I got it at Carquest but I’m sure NAPA or any other parts house can get it.