Best Radiator Stop Leak

Best Radiator Stop Leak

Even though most radiator stop leaks only offer a temporary fix, the fact that they can save you after getting stuck in the highway with a leaking radiator is a huge plus.

With multiple radiator stop leak options available, the only problem any car owner would have is finding one that works as advertised. There is no denying that a radiator stop leak (when used appropriately) can save you time and money.

But what is a radiator stop leak sealant and how exactly does it work? It’s also quite controversial and a lot of people will tell you to avoid it. Are they right?

Below we’ve listed our favorite stop leak sealants and answered your frequently asked questions. You’ll have a better idea by the end of the article what stop leak sealant is best for you and also what makes them work so effectively.

You’ll also understand exactly how they work miracles on leaks and can save you a fair few dollars.

What is a Radiator Stop Leak?

If you’ve never used one before, the name can be a little misleading. These substances can plug up leaks in your radiator, but not exclusively inside your radiator.

They circulate throughout your automobile’s cooling system. In the process, they target cracks or holes that result in lost coolant (antifreeze).

We’ll be delving into the science behind how they work in the next section. First, let’s cover how your vehicle’s cooling system functions.

This will help you better understand how these leak-fighting substances operate. For readers who are experts on your car’s internal workings, feel free to skip ahead.

Benefits of Radiator Stop Leak

  • Prevent a breakdown: Driving with a radiator leak is incredibly harmful to your vehicle. Any major loss of radiator fluid will likely cause your car to break down, leaving you stranded on the side of the road. The radiator works by cooling down the vehicle’s engine with the use of liquid coolant. The hotter the engine gets, the higher the chance it will overheat, causing a breakdown.
  • Improve performance: Checking to make sure the radiator stop leak is constantly getting fluid will help keep the car running properly. Over time, a leak will wear down your car’s power and could damage other parts of the vehicle.
  • Protect seals: Hardened or dried seals cause most oil leaks. Oil leak additives work by revitalizing the main seal and head gaskets to return to their original size and flexibility. This reduces the amount of oil that is capable of seeping through.

Types of Radiator Stop Leak

Years ago radiator stop leaks would essentially stick the leak back together using gummy solutions and whilst this did temporarily fix the problem, it also caused issues in that it would also clog the cooling system passageways and heater cores – not ideal! Vehicles have become more high-tech over the years and therefore so have radiator stop leak products.

There are many different options on the market which provide different features and knowing which is the right choice for you can be difficult. Stop leak formulas are now made from a variety of different chemicals that can provide a permanent seal to your leak.

Many of the products on the market have now been created with steel, aluminum, and copper which bind to make the seal. Liquid forms and tablet forms are available too – offering protection for varying levels of leakage. Check the material of your radiator and know your car and which fluids you use to maintain it before purchasing a radiator stop leak sealant.

How to Use Radiator Stop Leak

Once you’ve come to the conclusion that you need a radiator stop leak product for your leaking radiator or gasket, then you are going to want to keep a few things in mind from buying the product all the way to using the product.

Below are some of the steps required to use these products well and use them in a way that’s beneficial to your vehicle and actually stops the leaks in question.

Purchase the Appropriate Sealant: The first thing that you are going to want to do is find the best radiator sealant possible. This means one that not only meets quality standards but is designed for the vehicle you’re using it on.

Read Product’s Directions: It’s extremely important to read ALL of the directions included with your sealant. And you should not only read them but follow them exclusively. If the directions happen to disagree with any of our suggestions, then disregard what we have written here and follow the package’s directions.

Pour It Into The Radiator: Make sure that your product doesn’t require the radiator to be drained or flushed before it’s used. If it does, then that will need to be done before continuing.

Start the Car: Most of the sealant products on the market require the car to be idled or driven for 15 to 20-minutes before the product is able to work. So, start the car and allow the engine to warm up.

Allow The Car To Sit Overnight: One of the reasons many radiator stop leak repair jobs fail is that some people fail to let their car sit for the appropriate time.

Our Top Picks

1. Bar’s Leaks Liquid Aluminum Stop Leak

Bar’s Leaks Liquid Aluminum Stop Leak

The Liquid Aluminum Stop Leak from Bar’s is one of the best radiator stop leaks available and is super affordable to boot. It will seal leaks many types of leaks to a very high standard, including radiator leaks, freeze plug leaks, and gasket leaks. This product will also lower your water temperature. You can use it in all coolant systems, including all colors of coolant, antifreeze, and water.

Bar’s Leaks advise that you can add their Bar’s Leaks Liquid Aluminum Stop Leak straight into your cooling system, and after adding the liquid aluminum, to idle your engine for 10-20 minutes. This will allow the product to work. The liquid aluminum usually seals leaks within a few minutes. However, large leaks may require up to 20 minutes or may need you to add another dose of the Bar’s Leaks liquid aluminum stop leak.

  • Bar’s Leaks is compatible with all coolants
  • Add directly to your cooling system
  • Lowers water temperature
  • Works in up to 20 minutes

2. K-Seal Coolant Leak Repair

K-Seal Coolant Leak Repair

This K-Seal leak repair provides a tight seal to your cooling system quickly and efficiently. One of the best features is that it is simple to use and can be applied instantly to your coolant system. It’s capable of being added to either a hot or cold engine, saving your time. However, use extreme caution when mixing the solution with a hot engine to avoid hurting yourself.

Another benefit of K-Seal’s leak repair is it mixes with all types of antifreeze, so you need not worry about buying a specific brand for your car. It also can be left in the cooling system or flushed out if needed. K-Seal’s formula is designed to target holes and cracks then permanently seal them without damaging your engine.

While this may be an excellent radiator fluid stop leak, there are a few drawbacks you should be aware of. K-Seal does guarantee a permanent seal, but many consumers report they experienced some leaking again a few weeks or months after application. This could be due to the size of the crack, but it should still block small holes without issue. Another problem is it tends to clog easily and is a bit on the pricier side.

  • Tested to the highest standards to prove the effectiveness
  • Works within minutes of application
  • Safe on aluminum and plastic heads
  • Safe for all cooling systems and coolant types
  • Rejuvinating effects

3. ATP AT-205 Re-Seal Stops Leaks

ATP AT-205 Re-Seal Stops Leaks

If you’re looking for a safe and fast car radiator sealant then the ATP AT-205 Re-Seal product could be the right choice. This product is professional strength, so you know you’re getting some serious top-notch stuff when you invest in a bottle.

Not only does it stop leaks fast, but it also brings all your rubber seals & gaskets in the engine, transmission, power steering and hydraulic systems back to life with its rejuvenating formula. You won’t have to worry about flushing out the system either, as this radiator sealant is compatible with conventional and synthetic oils, power steering fluids, hydraulic oil, ATF and gear oil. Although the sealant is strong enough to totally restore worn-out or dry seals it won’t harm the internal components.

This is an easy-to-use product. All you need to do is to add the liquid into your car as you would normally with oil or brake fluid. The specially-formulated liquid is designed to blend with the oil and circulate when you vehicle is being operated.

The fluid is made from a plasticizer that essentially works by hardening and increasing flexibility. It gets to work in as little as five hours unless the seal or gasket is severely damaged.

  • One of the most professional radiator stop leaks
  • Safe for power steering systems and your heater core
  • Will seal leaks in a matter of hours
  • Can be used in a hydraulic system
  • Safe for all engine components

4. Prestone Radiator Sealer Stop Leak

Prestone Radiator Sealer Stop Leak

Here’s a brand you’ve probably heard of before. This leak stop formula is patented, and it acts immediately on contact with leaks in your radiator, water pump, and heating core. This liquid also protects your car from corrosion and pitting too.

The main difference between this stop leak product and some of the competitors is that this not only stops the leaks, but it finds corrosion points in your system and areas that are starting to break down. When it finds these spots, it seeks them out and fills them in before leaks happen. This helps you to get rid of immediate leaks and also prevent them from happening in the future.

The formula contains an ideal mixture of natural and synthetic fibers that can seal a small leak and moderate leaks. You also don’t have to worry about this one clogging your system.

  • Fixes corrosion on contact
  • Works with many different types of leaks
  • Doesn’t clog your system

5. Genuine Gm Cooling System Seal Tablet

Genuine Gm Cooling System Seal Tablet

These organic tablets will appeal to General Motors vehicle owners. They’re produced by ACDelco, which makes a range of GM-specific products and equipment.

If you want to take preventative measures against leaks, consider this product. They target porosity in metals and will stop up microscopic cracks before they escalate. The tablets are formulated to be safe for your coolant system. You can rest assured they won’t wear out plastic, metal, or aluminum.

They also have a range of protective capabilities. The first one is neutralizing pH: high acidity or alkalinity isn’t good for any of the parts in your engine. Secondly, corrosion and the formation of scale are inhibited. Finally, electrolysis is controlled.

Treating your coolant system is a breeze. You can either crush the tablets or drop them in. It’s recommended to flush and drain coolant systems that haven’t been cleaned in a while.

  • Won’t cause wear and tear on internal components
  • Protective abilities
  • Less toxic than other alternatives
  • Especially for General Motors engines
  • Reputable manufacturer


  • Consider the size of the leak in your radiator when adding a stop leak. You may not need very much for small leaks. While larger leaks may require much more. Very large leaks can be too much for the stop leak and will require professional assistance.
  • Wear appropriate protective gear while using a stop leak. Be careful not to get it on your skin or in your eyes or mouth. If you do, be sure to follow the safety instructions on the bottle.
  • To see if you’ve successfully stopped a leak, leave a piece of cardboard under the car where the fluid usually leaks. After filling the reservoir with stop leak, drive the car around for a bit, and then park it over the cardboard. Check the cardboard to see if any leakage took place.


Will Stop Leak harm my engine?

Put simply, no. However, it’s important to make sure you are buying the correct product for your needs, as not all radiator stop leaks are fully engine-safe in every capacity.

Some stop leaks are solely intended for use in the coolant system and are not formulated to be safe in the head unit or other engine components. So if your leak is coolant to oil, this will not work for you, as it may cause further damage or simply not bond effectively and, as such, not repair the leak.

The good news is that many are completely versatile and safe for your engine. Some, such as Bar’s leaks liquid copper that we reviewed at the number nine spot, are marketed towards use in lots of different systems. This means it’s very much safe for your engine and even able to provide leak sealant for things such as your head gasket or power steering pump.

How long does it take to see results?

Again, all stop leak sealants are different and some work faster than others. Some will need at least a few hours to work with a warm engine to seal – depending on the materials used in the sealer, while others claim to form a bond within just a few minutes. Check before buying.

What are the dosage recommendations?

Each product varies and will say on the packaging how much radiator stop leak sealant is required to fix the leaks. Dosage depends on your vehicle, the product used and also the size and number of leaks that you have. Some leaks will need several applications to form a strong enough bond. It’s always best to have more product in your garage than expected as you never know when you’ll need to quickly repair your car from a leak.

Are radiator stop leaks safe?

When using a radiator stop leak in the right way it can efficiently stop leaks in your system without causing any harm. This is not permanent, however, and you’ll eventually need to add more radiator stop leak liquid.

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