How to Safely Loosen a Stuck Bolt with Heat

Learn The Correct Way To Loosen a Stuck Bolt with Heat

I’ve dealt with my fair share of rusted bolts over the years, and I’m usually successfully using rust penetrant and vibration. I rarely resort to heat, but when penetrating oil, impact tools, and brute force fail, I turn to the torch. Knowing how to properly loosen a stuck bolt with heat can mean the difference between a smooth repair and snapping the bolt off entirely. But using heat isn’t just about blasting a flame onto metal—it requires a precise approach to avoid damaging surrounding components or weakening the metal.

Understand Why Some Heat Works But Too Much Can Damage the Fastener

To understand why rust penetrant, vibration, and heat work, it helps to think of rust as if it’s a glue or sealant. In other words, you have to break up the rust to get the bolt to move. Vibration and heat are two ways to break the rust.

Rust expands when heated, just like the base metal underneath. When you heat a stuck bolt, the metal expands in all directions, breaking the rust bond. The key is that the bolt heats and expands faster than the surrounding metal, breaking the grip that rust has on the threads.

But that’s not all—there’s a second trick to make this process even more effective. If you quickly cool the bolt after heating, it contracts rapidly, sometimes enough to create a gap between the bolt and the rusted threads. This thermal shock can be a game-changer when trying to loosen a stuck bolt with heat.

Once the rust bond is broken, rust penetrant can seep into the threads and loosen things further so you can back the bolt out.

But Too Much Heat Damages The Bolt

Most automotive bolts are Grade 5, while some critical load-bearing bolts are Grade 8. Grade 5 and Grade 8 bolts are made from medium carbon steel, typically 1045 or 1541 steel for Grade 5 and 4140 or 4340 for Grade 8. The tempering temperature for both grades is typically between 800°F to 1,100°F (425°C to 595°C). If you exceed those temperatures, you weaken the bolt by removing its temper. It can’t be reused after that.

Cherry red is simply too hot and completely unnecessary

I often see mechanics using an oxy-acetylene torch to heat stuck bolts to cherry red. That may make them feel manly, but that high heat is just unnecessary. The rust breaks at much lower temperatures, around 300–400°F (150–200°C). So, the additional heat doesn’t do a better job and damages the bolt.

How to Use Heat to Loosen a Stuck Bolt

• Clear the Area – Before applying heat, make sure there are no flammable materials nearby, including grease, oil, or fuel lines. If you’re working near sensitive components, use a heat-resistant blanket to protect them.

• Choose the Right Heat Source – A propane torch is hot enough for small fasteners. If you’re dealing with large, rusted suspension bolts, an oxy-acetylene torch provides the highest heat output.

• Apply Even Heat – Focus the flame on the bolt head and the exposed threads if accessible. Keep the torch moving to prevent overheating one spot, which could weaken the metal. Heat for about 30–60 seconds, depending on the size of the bolt.

• Try to Turn the Bolt – After heating, use a wrench or breaker bar to apply steady pressure. The expansion should have broken the rust’s grip, allowing you to turn the bolt. If it’s still stuck, move on to the next step.

• Use the Thermal Shock Trick – If heat alone isn’t enough, immediately spray the bolt with penetrating oil or water to cool it rapidly. This sudden contraction can help break rust’s hold. Be careful when using oil, as the bolt may still be hot enough to cause flames.

• Repeat if Necessary – Sometimes, it takes two or three cycles of heating and cooling to fully loosen a stuck bolt with heat. Don’t rush—let the process do its job.

How to use heat to remove a rusted bolt

Using Cold to Aid the Process

If you don’t have a torch, another approach is cold shock. Turning a can of computer dust spray upside-down will cause it to spray its propellant on the bolt. That will chill it, causing it to contract. When combined with heat (by alternating between heating and freezing), this method is especially effective.

The Dos and Don’ts of Using Heat on Stuck Bolts

DO:
✔ Use heat on thick metal components – Suspension arms, engine blocks, and frame bolts can handle heat well.
✔ Keep the flame moving – Avoid overheating one area to prevent metal damage.
✔ Apply penetrating oil after heating – The heat draws oil into the threads for better lubrication.
✔ Wear protective gear – Gloves, safety glasses, and flame-resistant clothing keep you safe.
✔ Let metal cool gradually if strength is critical – For structural bolts, avoid rapid cooling to prevent brittleness.

DON’T:
✖ Use heat on aluminum or thin metals – Aluminum melts at a much lower temperature than steel, and excessive heat can warp or crack it.
✖ Overheat high-strength bolts – Some bolts are heat-treated (tempered) for strength. Overheating can ruin this treatment, making them weaker.
✖ Apply heat near fuel lines, brake lines, or wiring – Fire hazards and material damage can occur.
✖ Hold a wrench on a bolt while heating – Metal expands when hot, so wait until it cools slightly before applying force.
✖ Cool too quickly if tempering is a concern – Quenching red-hot steel can alter its hardness, making it brittle.
Avoiding Metal Tempering Issues

WARNING: Many bolts, especially in automotive applications, are heat-treated for strength. Heating a tempered bolt beyond 600°F (315°C) can weaken it permanently. If a bolt turns blue or white-hot, you’ve likely exceeded the safe temperature.

©, 2025 Rick Muscoplat