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Rebuilding Your Heater Box

Keep out Mr. Freeze

Mike Petralia Jul 1, 1999

Step By Step

We cleaned and oiled our original control cables with TAL 15 spray lubricant until they worked like new. Replacement cables are available from National Parts Depot if yours can't be salvaged.

Once all seven firewall fasteners are removed and the heater hoses and fan motor have been disconnected, the case assembly can be removed as a unit. You may have to pry it off and scrape the area clean with a putty knife to remove the factory adhesive.

The heater core won't last forever, so most older cars will need a replacement during their lifetime. Ours didn't look too bad, but we decided to replace it just to be safe. New and rebuilt cores are available from sources like National Parts Depot for a reasonable price.

Use a cleaning solution to sanitize the outside of the case. Blow out the case using compressed air, and clean the inside as well.

Our heater case assembly had some minor surface rust that we sanded off and repainted.

To lubricate the motor's shaft, or if the fan needs replacing, it can be pried off using two screwdrivers (as shown). First, remove the retaining clip and gently pry up on each side, rocking the fan back and forth until it pops off. Ours was stubborn, so we were careful not to bend the thin steel fan.

National Parts Depot has replacement control cables for most early Chevrolets. The new cables don't come labeled from National Parts Depot, and all of them look the same except for length. So, we measured each new cable (foreground) against the old factory cable it replaced.

If you're working on an early Chevelle like this one, refer to the above photo to make sure you don't miss a bolt or cross thread them when removing and reinstalling the heater box. Only the five 7/16-inch nuts will interchange. A: Five 7/16-inch-head perimeter nuts. B: One 3/8-inch-head sheetmetal screw. C: One 7/16-inch head machine screw. B&C can only be accessed from inside the passenger-side wheelwell.

We oiled all the heater-box vent doors, spraying some TAL 15 on the hinge shafts while working the door back and forth.

Test the blower motor by grounding its case to the negative terminal of a spare battery and running a wire from the motor's spade connection to the positive battery terminal. It's a good idea to lube the motor shaft at this time, even if it doesn't squeak.

With the heater box out, this is a good time to replace the old firewall insulation pad. Push the old plastic clips securing the pad to the firewall in toward the interior to remove the pad. Our new insulation pad from Classic Industries included new clips.

Use black RTV silicone or 3M Weatherstrip Adhesive to secure and seal the case assembly on the firewall. Restoration specialists use common hardware store plumber’s putty to seal any large gaps between the firewall and the case assembly. Some of the bolts must be accessed from inside the wheelwell.

The heater core hidden deep inside your climate control system was not designed to last forever. The first sign of heater core failure is a puddle of sticky water on the carpet accompanied by the stench of warm antifreeze. When the heater core springs a leak, most people will just bypass it by splicing both heater hoses together, but replacing the heater core is not difficult and can be done in an afternoon.

This '66 Chevelle's heater hadn't been used in some time and was in need of a redo., so we ordered replacement heater system components from Classic Industries and National Parts Depot. The complete system was removed from the car for the overhaul, which included cleaning and lubricating the control cables and replacing the heater core.

We first unbolted and removed the passenger-side inner fenderwell in order to access the heater case assembly on the firewall. There are five 7/16-inch-head perimeter nuts, one 3/8-inch-head sheetmetal screw on the bottom left side, and one 7/16-inch-head machine screw up high on the right side of the motor bolting the case assembly in place. The 3/8 screw and 7/16 bolt had to be accessed from inside the passenger-side wheelwell. On early Camaros, this means removing the right front fender as well. All five 7/16 nuts can be unscrewed from the engine compartment. Drain the cooling system and remove both heater hoses if they're still connected at this time.

With the outer case assembly removed, you can pull the heater core box out from under the dash. It may take some interior gymnastics to get it out, and you may have to remove some factory brackets bolted to the firewall that help support it. Be sure to clean and lubricate all moving parts such as the hinges, control rods, and original cables if they're not broken. Hang the cables from one end and slowly drip light oil down the cable, moving it in and out of its sleeve to distribute the oil properly.

After the bench-top clean-up and rebuild, reinstall the case assembly, sealing it to the firewall with black RTV silicone or 3M Weatherstrip Adhesive. You should also flush the engine's cooling system and pour in a fresh 50/50 mix of antifreeze and water. Top that off with new heater hoses and you'll have a heater assembly that'll blow hot air like it's brand new.


Classic Industries
Huntington Beach, CA 92648
National Parts Depot - Florida
Ocala, FL 24474

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