Here is a good step by step
MID YEAR HEATER CORE
REPLACEMENT
By Randy Merzdorf
Imagine a picture perfect autumn day - brilliant sun lighting up the many hued leaves dancing through the air. The kind of day that would be even more perfect by getting the mid-year out of the garage once more before all those leaves turn to snowflakes. As you warm up the Corvette in your driveway, you reach for the heater fan switch to get the chill out and - uh oh. When was that rare steam heat option installed in your car? Backing the car into the driveway, you pop the hood and confirm your worst fears, Elvis and your heater core have left the building.
At this point, you have two choices - park it and worry about it next spring, or roll up your sleeves and replace the heater core. After reading over the shop manual, it didnÕt seem too bad of a job and I decided to tackle it. I did discover a lot of reading between the lines was a requirement, and figured I would record what I discovered with the hopes that someone else would learn from my mistakes. The process I went through was on a 1965 non-air car, although I suppose an air car and other mid years would be somewhat similar.
The first step is to clear a path to the heater core. Starting inside the car, you want to remove the following parts:
¥ Glove box light (unsnap front switch cover and plunger carefully, as a spring inside will guarantee flying parts everywhere)
¥ Glove box door, surround, and glove box (be careful not to dent door, as bottom hinge is spring loaded)
¥ Console surround panels
¥ Heater motor resistor connector (on top of ductwork directly under glove box)
¥ Control cables for heat and defroster (be careful not to damage push on cable retainers, which can be carefully removed with thin screwdrivers and needle nose pliers)
¥ Heater air deflector - this is behind the radio and is held in place by three screws
¥ Stamped Òspeed nutÓ above the accelerator pedal
Be sure to put anything you remove into labeled ziplock bags - you will be surprised how many pieces/parts you have already accumulated that might get lost. Now itÕs time to head out under the hood.
Drain coolant from the radiator. Make sure the radiator cap is loose or off, or nothing much will drain at all. Two gallons of coolant worked for me. If any other parts of your cooling system look suspicious, you might want to drain the whole system and take care of that as well. You also want to put some newspaper under the car before you remove the heater hoses. I took the top one off first and then the bottom one, but ended up dumping coolant on the floor anyway. You probably want to remove the expansion tank and battery, as this stuff will soon be in your way as well. After removing the blower motor connections, itÕs time to head back inside the car once again. Make certain that you put down plastic sheets, rags, etc. behind the ductwork and over the floor before the next step to minimize the amount of coolant running onto your nice original carpeting.
Head back out under the hood and remove the six speed nuts holding the blower motor housing to the firewall. Be aware that the four nuts closest to the center of the car are actually holding the interior ductwork via four studs. Now would be a good time to recruit a helper. Even though the inside ductwork is probably stuck to the firewall, why take a chance? Pull off the underhood ductwork - wiggle it slowly off the studs and donÕt force anything so as not to scratch or break the brittle fiberglass. The end with the blower motor needs to come off first to clear the angled heater core outlets on the other end. I did not have to take off the blower motor, and left this duct lay inside the fender. You can take this stuff out if you wish, but it seemed unnecessary to me. You probably do want to clean out the bushel of leaves you will probably find in the bottom of the duct.
Take off the two round seals around the heater core outlets and also check the condition of the seal around the hole in the firewall. If any of these are damaged, you need to order replacements. Some vendors sell a complete seal kit for the whole job with everything you need. You may want to also evaluate the other cooling system components at this time - hoses, clamps, etc. You may as well get rid of all the junk while everything is apart, so you donÕt have to go through this again.
Now we are back inside the car again (unless you have convinced your helper to hang around). Wiggle the heater box around until it pops loose from the firewall. Again, be very careful not to force anything - patience is your best friend here. Once loose, pull the ductwork straight back, and tip the top back toward you. Lower the right side first. Pull it out from behind the center console, and remove it from the car. Be careful to tip the assembly backward while you remove it, as you will be amazed how much coolant is still in the heater core. Set the assembly on the garage floor, heater core assembly up. Now would be a good time to take a beverage break.
Next, letÕs find the old seals from inside the car. You should find two of them - a black seal (which is probably stuck on the ductwork surrounding the heater core), and a larger gray seal that is either stuck to the firewall inside the car, or probably fell on the floor inside the car. Again, if these are in bad shape, you might want to replace all this stuff while everythingÕs apart.
Peeling off the black seal stuck to the heater box will reveal two rectangular metal clips over two of the studs. Carefully work these off using the thin screwdriver/needle nose pliers technique you have perfected in a previous step. This way, you can use these little critters over again. Once these are removed, you can remove a metal surround plate and heater core from the rest of the ductwork. Remove the four-ùÓ hex head screws that are holding the two straps that hold the heater core to the metal surround. Be careful not to tip the heater core until you can drain the remaining coolant, as there is STILL more coolant left in this thing (where is it all coming from?).
Now we are halfway home. We start cleaning up stuff and putting it back together again. The inside of all of the ductwork needs to be cleaned out, as it is probably wet with coolant. Note the condition of the foam seals on the heat and defroster doors; these will probably disintegrate when you try to remove the coolant that has undoubtedly soaked into them. Also, check both doors for binding and proper operation, as you donÕt want to take this stuff apart ever again. Clean up and paint the metal surround and the heater core retaining straps, if required. Fix any cracks found in the ductwork as required.
I found that my car still had the original dated Harrison core (which would explain why 30+ years later, it finally wore out). There are a few options when it comes to picking a new heater core. I went with an AC/Delco current replacement model. The outlets match the original in appearance, but not much else looks like the original. While replacing the heater core for my 1966 Impala (which went out about two weeks after the Corvette), I purchased a heater core through a local radiator shop. It came from a company called AAA Distributing (1-800-621-4349), and it was an absolute dead ringer for the original except for the embossed Harrison logo and date code. You might see if they have a similar configuration for a Corvette. The real die-hards might want to find a dated NOS unit - good luck! Unless we have to start taking apart ductwork for judging, any of the options above should work OK and give you toasty feet to boot.
Take the heater core of your choice, and attach it to the metal surround with the two straps and four-ùÓ hex head screws. Place the heater core/surround assembly inside the ductwork over the four studs and replace the rectangular metal clips over the two studs closest to the outlets. Place one of the rectangular black seals over the four studs and then place the larger gray seal over the top of the black seal you just installed.
Now its time to call your helper out of retirement, as the most aggravating part of this process is coming up. The ductwork is going back under the dash and behind the radio, back over the stud above the accelerator pedal, and heater hose outlets and four studs on the passenger side going partway through the firewall. Watch the gray seal inside the car as it wants to get all bunched up in this process and not seal properly.
While someone holds this assembly inside the car, you need to go back under the hood to put the other black firewall seal over the protruding studs, and the two round seals over the heater core outlets. Next, work the blower motor housing over the six studs under the hood. Again, be patient and wiggle parts around inside the car and under the hood as necessary until the studs and outlets go through the housing. It will take several tries to make this happen, so take your time. You may want to be sure that the fender emblem stud was cut off as per assembly manual instructions, or this will interfere in lining up the studs you canÕt see up under the fender.
Once everything is lined up, thread the six speed nuts over the studs. Get them snug, but donÕt tighten them yet. Before you tighten them, be sure to line up the ductwork behind the radio under the defroster outlet. The match should be close, but should not touch, as a squeak could occur here. Tighten the stud above the accelerator, and then the six studs under the hood. Get them pretty snug, but be careful not to overtighten them, as the blower motor housing can be easily cracked if you overdo this. After this step, another break for you and your helper is probably appropriate - especially if you want your helper to hang around.
Now we can finish up under the hood. Put the battery back, along with the expansion tank and the heater hoses. Be sure to drape the hose connected to the bottom outlet OVER the short piece of hose between the ÒTÓ fitting and the expansion tank. Check the assembly manual if in doubt of how this should look.
Inside the car, the heater deflector behind the radio can be reconnected, as can the heater and defroster control cables. Check for free movement of the cables, as well as the complete opening/closing of the corresponding doors at each end of the cable travel. Adjust the cable end clamping points as necessary. There are slots in the attaching brackets for just this purpose. DonÕt forget to replace the cable retaining clips over the posts when your adjustments are complete. The console side panels can go back too.
The glove box can go back in after you fish the glove box light socket through the opening in the top. Put the glove box light back together again. Now, the glove box door surround and the door itself can all be reattached. Again, donÕt forget the door hinge is spring loaded - you donÕt want the door to get away from you and get dented in the process. Be sure you donÕt have any unidentified pieces/parts left or missing. HereÕs where your art of bagging and labeling pays off!
Here comes the big finish! Refill the cooling system with approximately 2 gallons of antifreeze/water mix (it may take a couple of warm-up/cool down cycles to get all the coolant in and the air out). The expansion tank should be half full cold. Reattach the battery cables, start the car, and check for leaks.
Now you are ready for the next sunny autumn day of leaf chasing, knowing your trip can be made in cozy comfort complements of your newly refurbished heating system!
If you had not swapped the H/C out, you know that the old one would have failed shortly after putting that beast of an engine in...
That's Murphy at work...:L
