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1973-1987 Chevy & GMC Square Body Truck Heater Core Replacement

1973-1987 Chevy & GMC Square Body Truck Heater Core Replacement

Last Updated October 16, 2026 | Nate Moonis

A leaking or clogged heater core can leave your 1973-1987 Chevy or GMC truck without reliable cabin heat and may eventually lead to coolant inside the cab. Replacing the heater core requires removing the interior heater box, but the job can be completed with common hand tools and some careful disassembly.

Before getting started, gather your replacement heater core, any worn hoses or seals you plan to replace, and all of the tools you'll need. Taking care of these components while the heater box is apart can save you from having to repeat the job later.

Check out the video below for a more in-depth installation breakdown.

How to Replace a 1973-1987 Chevy or GMC Truck Heater Core

This step-by-step guide will walk you through draining the cooling system, disconnecting the heater hoses, removing the heater box, replacing the heater core, and reassembling the system. The exact fastener sizes and heater box components can vary depending on previous repairs and if your truck is equipped with air conditioning.

Tools and Supplies Needed:

  • Ratchet and socket set
  • 7/16-inch socket
  • 5/16-inch socket
  • 1/4-inch socket
  • Ratchet extensions
  • Swivel socket adapter
  • Pliers
  • Marker
  • Tape
  • Replacement heater core
  • Replacement foam weatherstrip or gasket material
  • Coolant
  • Distilled water
  • Drain pan

1973-1987 Chevy and GMC Truck Heater Core Installation

Step 1: Drain The Cooling System

Place a drain pan beneath the truck and drain the coolant from the radiator.

Step 2: Disconnect The Battery

Disconnect the battery before beginning the heater core removal.

Step 3: Remove The Heater Hose Clamps

Remove the clamps securing the heater hoses to the heater core connections at the firewall.

Important: Keep track of which hose connects to each heater core tube. The upper hose should typically have a 3/4-inch inside diameter and run toward the radiator, while the lower 5/8-inch hose generally runs toward the water pump or top of the engine.

A wide angle shot of a mechanic leaning over the front fender of a classic Chevy truck and reaching into the engine bay to remove a heater hose clamp

Step 4: Disconnect The Heater Hoses

Gently twist each heater hose to break it loose from the heater core tube, then pull the hose free.

Pro Tip: Avoid pulling straight back on a stuck hose. If necessary, carefully twist it with pliers. As a last resort, cut a short slit into the end of the hose, peel it away from the tube, and trim off only the damaged section if the hose is still suitable for reuse.

A wide angle shot of a mechanic leaning over the front fender of a classic Chevy truck and holding a disconnected heater hose up to the camera and pointing at it

Step 5: Remove The Lower Heater Box Nut

Use a 7/16-inch socket, several extensions, and a swivel adapter to reach and remove the difficult lower heater box nut between the inner fender and A/C case.

Pro Tip: Wrap a small amount of tape around the swivel adapter to keep it from flopping around as you guide the socket onto the nut.

A close up of a swivel socket removing the lower heater box nut on a classic Chevy truck

Step 6: Remove The Remaining Firewall Nuts

Remove the remaining heater box mounting nuts from the engine compartment. One is located toward the top of the box with another positioned below it.

A mechanic using a socket wrench to remove the remaining heater box nuts

Step 7: Remove The Glove Box

Move inside the cab and remove the screws along the bottom of the glove box, then remove the glove box to gain access to the heater assembly.

A mechanic sitting in the cab of a classic Chevy truck while gesturing at where the glove box was before removal

Step 8: Remove The Heater Box Fasteners

Remove the accessible screws securing the heater box and surrounding components. These fasteners are typically 5/16 inch, although previous repairs may have resulted in different hardware being used.

A close up of the underside of the dash in a classic Chevy truck with a mechanic using a socket wrench to remove the heater box fasteners

Step 9: Remove The Remaining Heater Box Mounting Bolt

With the surrounding pieces removed, locate and remove the remaining heater box mounting bolt.

A close up of the underside of the glove box area with a mechanic using a socket wrench and an extension to remove the last heater box mounting bolt

Step 10: Remove The A/C Components

If your truck is equipped with air conditioning, remove the additional A/C components and fasteners obstructing removal of the heater box.

Note: Trucks without factory air conditioning require less disassembly during this portion of the installation.

A mechanic sitting in the bed of the truck holding black plastic air conditioning components while looking at the camera

Step 11: Label The Vacuum Hoses And Control Cables

Label the vacuum hoses before disconnecting them so each hose can be returned to its original location. Mark the positions of the heater control cables with a marker before removing them.

A close up of a mechanic using a black marker to mark the positions of the heater control cables before removing them

Step 12: Disconnect The Control Cables

Remove the small retaining clips securing the control cables and disconnect the cables from the heater box.

Important: Keep track of the small cable retaining clips during removal. They can be easily lost.

A mechanic holding the small control cable retaining clip up to the camera after removal

Step 13: Disconnect The Vacuum Hoses

Slide the retaining clip off the vacuum hose connections and carefully disconnect the vacuum hoses from the heater box.

A mechanic using his hands to carefully disconnect the vacuum hoses from the heater box

Step 14: Remove The Heater Box

Carefully pull the heater box away from the firewall and remove it from the truck.

Important: Inspect the heater box for rodent nests or droppings. If contamination is present, wear appropriate respiratory protection and thoroughly clean the heater box before continuing.

A mechanic sitting in the cab of a classic Chevy truck while pulling the heater box out from under the dashboard

Step 15: Remove The Heater Box Cover

Remove the three upper screws and four corner screws securing the heater box cover, then separate the cover from the assembly.

A close up of a mechanic using a screwdriver to remove the heater box cover on the side of the assembly

Step 16: Inspect The Heater Box Components

Inspect the vacuum actuator, internal door, rubber seals, and surrounding components while the heater box is apart. Make sure the door operates correctly and replace any damaged components before reassembly.

Inspecting the internal components and vacuum actuator of a classic Chevy or GMC truck heater box

Step 17: Remove The Heater Core Assembly

Slide the heater core assembly out of the heater box through the opening in the housing.

A mechanic standing in front of a table removing the heater core assembly from the top of the heater core box

Step 18: Remove The Heater Core Brackets

Remove the brackets securing the old heater core to the assembly. The demonstrated truck uses 5/16-inch and 1/4-inch fasteners.

A mechanic standing in front of a table holding the heater core assembly uses his hands to remove the heater core brakets

Step 19: Install The New Heater Core

Position the replacement heater core in the assembly and reinstall the mounting brackets to secure it.

A mechanic standing in front of a table holding the heater core assembly and sliding the new heater core into place on the assembly

Step 20: Replace The Heater Box Gasket

Inspect the gasket separating the hot and cold air passages. If the original gasket has deteriorated, cut replacement foam weatherstrip to size and install it in the original location.

A mechanic standing in front of a table holding the heater core assembly and inserting the new gasket into place inside the assembly

Step 21: Reinstall The Heater Core Assembly

Slide the heater core assembly back into the heater box and begin reinstalling the screws that secure the housing.

A mechanic standing in front of a table holding the heater core assembly and sliding the heater core assembly back into the heater core box

Step 22: Reinstall And Adjust The Control Cable Bracket

Reinstall the slotted control cable bracket in its original position using the marks made during disassembly as a guide.

A close up of a mechanic centering the control cable bracket on the side of the heater core box

Step 23: Test The Heater Control Door

Reconnect the control cable and operate the HVAC controls to verify that the internal door opens and closes through its proper range.

A mechanic using his fingers to open and close the heater control door to ensure proper movement

Step 24: Reconnect The Vacuum Lines And Cables

Reconnect the vacuum lines and control cables according to the labels and marks made during disassembly.

A mechanic sitting the cab of a classic Chevy truck and using his hands to reconnect the vacuum lines and cables underneath the dashboard in the passenger side footwell

Step 25: Position The Heater Box Against The Firewall

Guide the heater box back into position against the firewall, making sure the cables, vacuum hoses, and electrical connections don't become trapped behind the housing.

Note: The firewall sound-deadening pad also helps seal the heater box against the firewall. If the pad is missing or damaged, the video recommends adding an appropriate gasket sealer to help seal this area.

A close up of a mechanic gently pushing the heater box into place against the firewall underneath the dashboard

Step 26: Secure The Heater Box

Align the heater box with the firewall mounting holes and start the upper mounting nut first. Align the remaining studs and fasteners, then tighten the heater box into place.

Pro Tip: For the difficult lower firewall nut, lightly tape the nut inside the socket so it doesn't fall out while you guide the extensions into position.

A close up of a mechanic hand tightening the heater box mounting nuts

Step 27: Reinstall The A/C Components

For A/C-equipped trucks, reinstall the previously removed air conditioning components and secure them with their original fasteners.

A mechanic sitting in the cab of a classic Chevy truck slotting the A/C components into place underneath the dashboard

Step 28: Reconnect The Heater Hoses

Reconnect the heater hoses to their original heater core tubes and reinstall the hose clamps.

Step 29: Refill The Cooling System

Refill the cooling system with the appropriate mixture of coolant and distilled water. The video recommends approximately a 50/50 coolant-to-water mixture.

Important: Use distilled water rather than tap water to help reduce mineral deposits and corrosion inside the cooling system.

A mechanic leaning over the engine bay of a classic Chevy truck while pouring distilled water into the radiator

Step 30: Bleed The Cooling System

Reconnect the battery, start the engine, and turn the heater on. Allow the engine to reach operating temperature so the thermostat opens and coolant circulates through the radiator and heater core. Continue checking the coolant level as trapped air works its way out of the system.

Pro Tip: After driving the truck for about a day, allow the engine to cool completely and check the coolant level again. Additional trapped air may have worked its way out of the system.

Congratulations, you're all done!

Restore The Heat In Your 1973-1987 Chevy or GMC Truck

A fresh heater core can get your classic Chevy or GMC truck's heating system working properly again while giving you an opportunity to inspect the hoses, seals, vacuum components, and heater controls hidden inside the HVAC system. Taking care of worn components while everything is apart can also help prevent another round of disassembly later.

Keep your classic truck comfortable year-round with replacement heating and cooling components from CJ Pony Parts.

About the Author
Nate is a passionate writer and turbo four-cylinder fanatic. He uses his love for performance vehicles and automotive history to create content for experts and beginners alike.
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