This thread is about how to take the heater box out of a 73-79 dentside, w/o air conditioning, and modifying the box to accept the larger, high out-put heater core. Bronco's of this era come with the high out-put cores. When you remove the heater box and the core removal plate, you can see Ford left room for a larger core, but changed the inner core holder to accept a normal size or high out-put core.



If you want to install a H.O. core, you have to grind off the rivets on the steel plate side of the box and remove the plate. I saw a guy on youtube, Visegrip garage or something like that, where he just reached inside the inspection cover and broke the saddle or core retainer so that he could drop a h.o. inside. That is so stupid because that holder for the core also channels air either to the core or to heat the air, or cool air, and if you just break it off your heater will be always mixing hot and cold air. I'll show you what I mean in a photo further along. Grind off the rivets and remove the steel backing plate. You can use machine screws to reassemble later.



Once the back is removed, you can see the air channels I mentioned. The red circle on top is where the fan sits. The red circle about in the middle is the 'saddle' that hold the core in place. The red arrows show the flow path of hot air. The blue ones show cold air. With that in mind, it is easy to see that you just break the saddle off, you risk damaging the case of the box, and also you might allow cold and hot air to mix.



I found that I was able to put a small grinding disk on a drill and cut the saddle off along the edge of the blue tape. Once it was free, I had to grind down the remining ridge of the saddle because I was not able to cut 100% flush. Once it was free, I moved it at about 45* to where it was, checked the fit of the h.o. core, traced the position with a sharpie, and glued it back into place with JB Weld.




Here is the new position of the core saddle.



And here is the new, h.o. core in its new home. All it needs is a bit of foam around it to prevent air from passing w/o going through the core.


You'll have to find a way to hold it in place while the JB weld is drying. I placed one of these electrical outlet spacers (they fold and click together, so you can make them the thickness you need) under the bottom of the saddle to hold it up to the propper height so that it touches the steel backing plate when in place. I also cut piece of pipe insulation foam to keep the glued piece in the proper place while drying.





Here is the normal and high out-put cores.

 

Everything is removed, cleaned, wire-wheeled and painted, and then put back together with machine screws and lock nuts. BTW, I notice that the 78 and 79 dents had the high out-put heater cores from the factory. My truck has one now too!


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Even after 47 years the rubber around the heater diverters is still good. They just don't make things like they used to, right?

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I even managed to get this nifty diverter that clips on the bottom of my box and channels the hot air in different directions. I never had one. Now I do.
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*for those rebuilding their box, the trickiest part is sealing the defrost vent to the box. Unless your dash is not in place, once you install the box you'll find you have 1/2" gap between the vent and the box. It is very hard to get something in there to seal it because there is zero room. I took some 1" foam and attached it with rubber cement to the box around the perimeter of the defrost port. That way I was able to guide the vent into place inside the foam so that it sealed up. Even the kits don't sell this piece. Good luck.

 

I've been looking for confirmation this would work and a writeup on how to do it successfully for *years*. This is epic! Never again shall I need to dress like an arctic explorer just to drive my truck on a cold day.

 

I hope it helps. It was a fairly easy process. The most difficult part was finding the seals for the ducts that attach to the body of the heater. I had to make a couple of them since they were impossible to find.

 

I know the box is very different, but does this apply to the factory AC setup as well? I have a 73 with factory AC. Wondering if it has the small heater core or not?

 

I could be wrong, but I believe that the AC heater core is the same as the high output.

 

A mistake that I made was not to seal really well between the core sides, top and bottom with foam. If air can bypass the core it will, and that will make a less efficient heater. I mean that your hot air will be less hot than it could be. Lowes sells rolls of cylindrical foam. Just pack it in on the sides, and use flat strips for the top and bottom.