How to Plumb a Radiator Overflow Tank on a Mopar

If you’re restoring or maintaining a classic Mopar—whether it’s a ’68 Charger, a ’70 Challenger, or a vintage Plymouth—you know that keeping the engine cool is non-negotiable. One of the most common yet overlooked upgrades? How to plumb a radiator overflow tank on a Mopar. Without it, your cooling system can lose coolant, overheat, or even suffer internal damage. In this guide, we’ll walk you through the entire process with clear, beginner-friendly steps, expert tips, and real-world insights to keep your muscle car running smoothly.

Why Do You Need an Overflow Tank on a Mopar?

Many older Mopars (pre-1970s) were built without a coolant recovery system. Instead, excess coolant simply spilled onto the ground when the engine heated up—a wasteful and potentially dangerous setup.

According to the Society of Automotive Engineers (SAE), up to 15% of coolant loss in older vehicles stems from unvented expansion during normal operation. An overflow (or recovery) tank solves this by capturing expelled coolant and returning it to the radiator as the system cools—maintaining optimal fluid levels and pressure.

💡 Fun fact: Modern vehicles use pressurized closed-loop systems, but classic Mopars benefit immensely from retrofitting a simple overflow tank.

What’s the Difference Between an Overflow Tank and a Recovery Tank?

This is a frequent point of confusion—and getting it wrong can lead to poor performance or airlocks.

For most Mopar enthusiasts, a recovery tank is the smarter choice—it creates a semi-closed system that mimics modern engineering while staying true to classic aesthetics.

🔗 Learn more about coolant expansion and thermal dynamics on Wikipedia.

Tools & Materials You’ll Need

Before you start, gather these essentials:

• Overflow/recovery tank (universal or Mopar-specific, e.g., from Summit Racing or Be Cool)
• 3/8″ or 1/2″ high-temp coolant hose (rated for 250°F+)
• Radiator cap with recovery valve (typically 15–18 psi for Mopars)
• Hose clamps (stainless steel preferred)
• Drill & mounting hardware (if fabricating brackets)
• Coolant (50/50 mix of ethylene glycol and distilled water)
• Safety gear: gloves, goggles, drip pan

✅ Pro Tip: Use silicone coolant hoses—they resist cracking, handle heat better, and look great in engine bays.

Step-by-Step: How to Plumb a Radiator Overflow Tank on a Mopar

Follow these precise steps to ensure a leak-free, efficient setup:

Step 1: Drain Partial Coolant (Optional but Recommended)

• Let the engine cool completely.
• Drain 1–2 quarts of coolant into a clean container to lower the level below the radiator neck. This prevents spills when removing the cap.

Step 2: Replace the Radiator Cap

• Remove the old cap.
• Install a recovery-style radiator cap (e.g., Stant #10257 or OEM Mopar equivalent). This cap has a second valve that allows coolant to flow back from the tank.

Step 3: Mount the Overflow Tank

• Choose a location away from exhaust manifolds, belts, or moving parts.
• Common spots: inner fender well, core support, or near the battery.
• Use brackets or adhesive mounts—ensure it’s higher than the radiator cap for proper siphon action.

⚠️ Warning: If the tank sits lower than the radiator, coolant won’t return properly, defeating the purpose.

Step 4: Connect the Hose

• Run a 3/8″ high-temp hose from the radiator’s overflow nipple (or drill/tap the filler neck if needed) to the tank’s inlet.
• Keep the hose as short and straight as possible—avoid sharp bends.
• Secure both ends with tightened hose clamps.

Step 5: Fill the System Correctly

• Fill the radiator to the top (when cold).
• Fill the overflow tank to the “COLD” mark (usually 1/3 full).
• Start the engine and let it reach operating temperature (~195–210°F).
• As it heats, coolant will expand into the tank. When cooled, it should draw back in.

📏 Ideal fill level: 1.5–2 inches of coolant in the tank when cold.

Common Mistakes to Avoid

Even experienced builders slip up here:

1. Using a non-recovery radiator cap → No coolant return = dry radiator.
2. Mounting the tank too low → Gravity prevents reflow.
3. Using fuel line or heater hose → Not rated for constant coolant exposure; degrades fast.
4. Overfilling the radiator → Leaves no room for expansion, causing immediate overflow.

Performance Benefits: Real Results

A 2023 survey by Mopar Muscle Magazine found that 78% of readers who added a recovery tank reported fewer overheating incidents, especially in stop-and-go traffic or during summer cruises. One reader in Phoenix noted his ’69 Road Runner’s coolant temp dropped 8–10°F after installation—simply by maintaining consistent fluid levels.

FAQ Section

Q1: Can I use a universal overflow tank on my Mopar?

Yes! Most universal tanks (like those from AFCO or Moroso) work fine. Just ensure it’s compatible with your radiator cap type and has enough capacity (16–24 oz is ideal).

Q2: Do I need to drill my radiator to add an overflow?

Not always. Many Mopar radiators have a pre-existing overflow nipple. If yours doesn’t, you can:

• Use a radiator filler neck with a built-in port, or
• Carefully drill and tap the existing neck (use a 1/4″ NPT tap).

Q3: Should the overflow tank be pressurized?

No. The tank must remain vented to atmosphere. Only the radiator is pressurized. A sealed tank can cause pressure buildup and hose failure.

Q4: How much coolant should be in the overflow tank when cold?

Fill to the “COLD” line—typically 1/3 full. Never fill it to the top; it needs room to accept expanding coolant.

Q5: Can I run without an overflow tank?

Technically yes—but you’ll lose coolant over time, risk air pockets, and increase the chance of overheating. It’s a low-cost, high-impact upgrade.

Q6: What’s the best overflow tank for a show-quality Mopar?

Look for polished aluminum tanks with Mopar-logo caps (e.g., from Be Cool or Griffin). They blend performance with period-correct style.

Conclusion

Knowing how to plumb a radiator overflow tank on a Mopar isn’t just about following steps—it’s about protecting your investment, enhancing reliability, and honoring the legacy of American muscle. Whether you’re cruising Route 66 or showing off at Carlisle, a properly plumbed cooling system keeps your engine safe and your pride intact.

✅ You’ve just upgraded your Mopar like a pro.
Now, share this guide with fellow gearheads on Facebook, Reddit (r/Mopar), or your local car club—because every classic deserves to run cool and clean!

Got questions or your own overflow tank hack? Drop a comment below—we love hearing from the Mopar community!