I took my heater out again, because the core was leaking. I have a new core to put in. Question to anyone who has done this, inside the case there is a gasket the core nozzles go through before exiting the case. Mine is shot. If the gasket is left off, will it hurt anything? It seems to be there to keep the core steady, but I could use something else for that. It is a pain removing the heater and I do not want to do it again.

Or cheap like home a/c foam rubber seal found at hardware stores. If needed held in pace use 3M weatherstrip adhesive. I've cut and made many car HVAC gaskets out of that stuff. It's also handy for use in the air filter case crankcase vent filter, cut proper thickness or double it up and squeeze it in the plastic holder there. Replaces a $2-$3 part for a few cents. Use a razor blade to cut it to shape and does not have to be exact, the squeezability of it makes it conform to the shape needed. $5 worth of the seal may well go for years......... ..

Check it by holding it to mouth and sucking through it, there is both closed cell and open cell foam. Closed will not pass air, you don't want that on crankcase breather but useable on the HVAC. Using open cell right now on both my Focus breathers.