dick,

I can't answer your question myself, however checkout FO_FDYFO's explination here.

Good Luck.

 

Heater core replacement for a 70- 72 chevelle:

This is the quickest way to do this. This applies only to cars with ac I don't know anything about non ac. you can remove your passenger side inner fender well or do what I did. There are 2 lower bolts that hold the inner fender well to the firewall. Mark a center point 7" straight up from the top bolt. Drill a 1" hole thru the metal or plastic. This will gain you access to one of the 4 heater core box nuts with a socket wrench. You can afterwards go get a 1" plug from a hardware store. This is sure better then taking off the fender and fender well if you can live with a whole in your well. I think gm should have put an access hole there to start with. Anyway after you get that nut off (and it is a nut not a bolt! the nut goes to a stud attached to the inner heater box.) the second one is right under the heater hoses and the other 2 are towards the engine and can be reached. Next remove dash pad (bunch of screws pointing up; don't forget the ones in the glove box). Then remove the center ac vent this is above the ac & heat controls, this lets you remove the plastic duct box like thing too. Then remove the passenger side foot air duct piece from the bottom of the dash. Remove the door jam sill plate so you can remove the kick panel. Squirt water thru your heater core or cap the ends because fluid will spill out on the next step. Now you are ready to pull the inner heater box away from the fire wall and let it rest on the floor. Undo 3 screws on top side of heater box. Where a control cable attaches. Leave the box sitting right there. Don’t even try to pull it out. Reach behind it and remove 4 screws that hold a backing plate on. Use a flash light and mirror if necessary. When they are out the backing plate with the heater core attached to it comes out. 4 more screws hold the heater core to the backing plate. Reverse the order with the new core. I would check the new core first for leaks if you can. I put it in line with my heater hose and ran the car to temperature to high temp pressure test mine. You don't want to do this twice. GM must have built the car around the core. I guess that why they call it a core. Hope it helps.
pete

 

An alternative to drilling a hole in your inner fender is to take off just the inner fender. With the wheel off, and after loosening about a zillion bolts, the inner fender will come out without removing the outer fender. Heater core replacement is not the worst job to do, but not a lot of fun either.

 

I'd only add to remove the battery before pulling the inner fender off. Even though the inner fender doesn't need to come out all the way, loosening it up and moving it out of the way works.
Really not that bad to do. I usually get an extra hand to get the box back into place on the firewall. Easier if someone helps guide it/watches it from the outside and starts the nuts back on it.