After replying to someone on (oddly) a '70 Chevelle SS Facebook page earlier today, I felt it was time to create a thread devoted to removing and installing the heater core in your 1968-1972 NON air Skylark or GS. First things first: it's important to try as hard as possible to source a radiator repair shop local to you. The factory fitment & function of the Harrison is second to none. I understand radiator repair shops have become a lost craft but if you have one local, use it and insist on the high efficiency cores from Maine Auto Radiator in Lewiston, ME. As with everything, the prices have gone up, so expect to pay in the $250 ish range or even a little more. That's how much the last one I had done cost earlier this year. My radiator guy - who recently sold his shop but will (hopefully) be doing radiator work on the side this spring - used these cores on close to a dozen OEM heater cores and radiators for me over the last few years and those that have been installed have worked flawlessly. You can roll the dice with a made in Mexico/China reproduction core - and it may never fail - but it's not a job you'll want to do more than once, if you can avoid it and the repros are prone to lousy tube fitment and leaks over time. Once you've figured out which direction you'll go in terms of a new core, it's time to get down to business on this two person job. After you've drained the coolant from the system, you're going to want to make as much room as possible in that front passenger's side area. I ended up pulling the seat out. Taking into account how much time you're going to spend on your back, I recommend removing it. The dash pad, glove box liner and (eventually) the radio came out as well. In retrospect, I'm glad I made as much room as I did. After you've made enough room to be comfortably uncomfortable, it's time to tackle the 5 speed nuts on the firewall side of the heater box. The long standing myth is that the inner fender needs to be removed or a hole drilled in the vicinity of that annoying "hidden" speed nut that sits behind the top of it. In fact, I recall reading in one of the factory manuals where it was recommended to drill a small hole as I've previously mentioned. I'm not interested in drilling any holes in my car but you may be - who knows? My resolution was this rachet/wrench and patience. It will come out, you just have to take your time. The other 4 speed nuts are easily accessible. There are also 5 pieces of round foam called heater stud seals that (I think) I got from Ames Pontiac but didn't use because I didn't remove the heater box on the firewall side. It's best to have all your ducks in a row when doing this and that's what I was trying to do. Now that the heater box is loose , the heater hoses are disconnected, the heat duct under the dash is removed and the heater cables have been carefully removed from the heater box, you and your helper are ready to pull the entire assembly into the passenger side footwell. Non air cars will have that pesky blue tab that wraps around the cable and affixes to the top of the heater box. It invariably always breaks, more so because it's the one that is used most often, that opens and closes the door in the heater box. The red one above is typically fine and the fix for the blue one that will break is a cable clamp like this: Once the box is out, it's time to clean 50 plus years of dried up goop and cracked foam on both sides of the actuator door. After you've scraped everything off, you'll need some 3M strip caulking for the section the new core sits in inside the heater box. You're also going to need a sheet of 1/4 foam for the little door. The bad news is: you're going to need to figure out a way to get a piece of that foam on the backside of said door. You can leave it off but be prepared to hear a "thud" each time you move that cable to shut the door. If you have small hands, you'll struggle but it'll be doable. If not, you may want to ask the Google machine how to become a contortionist. I used Weldwood contact cement brushed on both pieces of foam. You want to cut it just a bit bigger than the actual size of the door and remember, you only get maybe one or two stabs at affixing the foam to the backside of the door. Be patient. Once your new foam is glued back on and the caulking strip - which is very easy to manipulate - is sitting in the heater box, it's time to secure the new core in place. And now you're ready to reverse everything I've just typed. You and your helper will now have the incredibly enjoyable task of maneuvering the cumbersome, refurbished box and shiny heater core tubes under the dash and through the firewall - with limited space to move around - while trying to make sure you don't bend said tubes. It really is fun for the whole family. Once the box is back in, you're going to hook the cables back up to the heater box. Hopefully all three of the aforementioned plastic tabs are still intact. If not, at least now you have a reference. That annoying speed nut goes back on with some dum dum on your fingers to start it onto the stud. Once you start it, you can use the wrench to snug it up to the firewall.