67 GTO Heater core replacement
 

Is it difficult to replace the heater core?

I suppose the shop manual has instructions?

it is not the easiest thing to do, I used to remove the passenger side inner wheel well, that makes getting at the outboard bolts & nuts a lot easier. If you have never done one before, plan on it taking an weekend, if your good it will only be 6-8 hours the 1st time. Start to finish.
But it is a straight forward job, the manual should have a good section for it as it was a common job to do back in the day.

Or....if you just drive it on Sunny Days in the Summah.....just by pass it.............

I think the bigger question is, do you have A/C or not?

Without A/C, the swap is easy and straightforward. If I remember correctly, there are only 4 studs that go thru the firewall holding the heater core box under the dash. I was able to remove it without pulling the fender/inner fender.

With A/C, it takes a little more work since it's hard (almost impossible) to get to one of the nuts on the engine compartment side without pulling the fender & inner fender. Year's ago, I went so far as to take a hole saw to the inner fender to access the nut. Today I would simply pull the fender/inner fender.

Hope that helps. :)

Excellent point...that is exactly the current status... bypassed. I've only had a the car a year but wanted to drive a few times during the winter so heat would be nice next time.

No AC
 

That is a big help. No AC so maybe wouldn't be too hard. So, remove the 4 studs through the firewall? And the heater box is free?

Maybe a cable or two from the heater controls?

I believe there are actually 6 studs, but the only difficult one is the lower outside one behind the inner fender. It's hard to reach but it can certainly be done without removing the inner fender. Then there are 3 cables to remove on the inside, one for temp control, one for the floor heat door, and one for the defrost door. And that's it; it will come right out.

My hat is off to anyone who managed to get to that stud without removing the inner fender. I tried every contortion I could think off before throwing in the towel.

'67 with A/C, I've owned since 1981. I've changed it twice without pulling the inner fender. I don't remember it being a terrible job, and I'm not one of these guys who underestimates the amount of time it takes to do a job.

Usually when I see someone post "it should take X hours" here, I safely assume that I can double or triple the time.

In all fairness, I was considerably younger, thinner and more flexible when I did it.

Ha Ha, so was I Richard!

Core
 

I should have mentioned this in my initial post...

I bought the car a year ago. The heater core was bypassed and the previous owner didn't know the condition of the core.

But I would assume if it's bypassed, then it leaks? Does anyone bypass a good heater core?

Will

A heater core will generate unwanted heat if a door isn't 100% closed. So even though it's unlikely, it IS possible. You can pressure test the core while it is in-car. I expect it a do it yourself test would be straightforward. I'm thinking inner tube from a ten speed bike and soapy water to make sure these connections aren't leaking. You obviously don't want to over-pressurize it but higher than normal, like a couple of psi over your radiator cap's rating. Testing it removed would be preferrable as it doesn't need a very large leak to ruin your carpet. Micro leak could cause moisture and smell. And a radiator shop could repair it (depends on where it's leaking from) if it seems solid otherwise. It's a gamble, but *some* new parts are clearly inferior. Dunno.

I replaced the heater core in my 66.... all by myself. It wasn't much fun but it wasn't all that hard to do. The hardest part was getting those rubber plugs back thru the firewall to hold the insulation in. IF you do change it out O'Rielly has the best price of any for the heater core. When I did mine it was 39.95 plus the ride. I couldn't pass that price up, And it fit perfect. Plus it was a lot easier to repaint the housing too.

Here's a capture of the page from my 66 Tempest Shop Manual (reproduction) and their recommendation on how to deal with that troublesome inner fender. BTW on the next page they do instruct you to patch the hole.

No need to cut anything to get at that nut. All you need is a small wrench.... a ratcheting box end wrench will work. You just have to work your hand in.. left one works....and carefully get the nut off and remove it. I've done it twice with no problems. If you have to, put a small strong magnet on the wrench so you don't lose the nut.