Best Radiator Stop Leak: Protect Your Engine and Keep Your Car Running Smoothly

It sounds like magic, but it works and it's remarkable.

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BY Andra DelMonico , Mike Viera / LAST UPDATED ON September 30, 2021

Seeing a puddle of coolant under your car is never a good thing. Whether it’s coming from your radiator, water pump, heater core, freeze plugs, or other portion of your cooling system, it can mean big trouble for your vehicle if you don’t attend to it quickly. Luckily, there is a way to stop the flow of leaks without a costly, immediate repair. Radiator stop leak is designed to stem the flow of liquid out of your cooling system without blocking those narrow passages so vital to the proper circulation of coolant.


Available in a variety of liquid, pellet, and powder forms, stop-leak products are designed to be poured into your radiator where they circulate with your coolant to seal leaks. Additionally, most offer lubrication protection for your water pump and seals. Stop leak can be a very easy and inexpensive way to avoid a big headache; here are some of our favorites.

Best Overall
K-Seal Coolant Leak Repair

A tight sealer for your cooling system, it is simple to use and can be applied instantly to your radiator. It’s capable of being added to both hot and cold engines.

  • Mixes with all types of antifreeze
  • Can be left in the cooling system 
  • Experience leaking again a few weeks or months
  • Clogging
Best Overall
K-Seal Coolant Leak Repair
Best Value
Bar's Leaks Radiator and Heater Core Stop Leak

The compound works great at sealing leaks but also conditions the engine with liquid aluminum. It is also fitted with an Xtreme Cool formula that stops overheating and reduces water temperature in the radiator.

  • Seals leaks in plastic, aluminum, and metal
  • Can be added to the cooling system
  • Wait up to 20 minutes with the engine idling
  • May have to buy in bulk
Best Value
Bar's Leaks Radiator and Heater Core Stop Leak
Honorable Mention
Blue Magic Radiator and Block Sealer

This copper-colored radiator stop leak is designed for repairing and bonding to automotive, industrial, farm, and marine equipment. It has a liquid glass and copper formula to form a permanent seal.

  • 5-minute application  
  • Withstands up to 2000 degrees Fahrenheit
  • Best for large capacity systems 
  • Doesn’t dry clear
Honorable Mention
Blue Magic Radiator and Block Sealer

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Benefits of Radiator Stop Leak

  • Prevent a breakdown. When your radiator leaks, it loses fluid, and its ability to keep your engine cool. This allows excessive heat to build up, which can damage your engine. Prolonged exposure to increased temperatures will lead to an eventual breakdown. 
  • Prolong major repairs. If you aren’t able to take on the expense of a radiator replacement, then a stop leak can be the answer. It will fill the cracks and prevent leaks while you make arrangements for a radiator replacement. 
  • Extend the life of your car. Older vehicles tend to have more leaks. There’s a point when investing more in a complete radiator replacement isn’t worth it. However, you can’t buy a new car either. Stop leak prolongs the drivable life of your car so that it can keep giving you faithful service

Types of Radiator Stop Leaks


You will see powder stop leaks that come in different formulations and amounts. However, they all generally function the same. Pour the powder into your radiator and let it get to work. This option is simple and easy to use. However, it tends to be less effective than other solutions because you need enough powder to mix in without being so much that it clogs your radiator. 


This type of stop leak isn’t a popular one, but you will still be able to find a few options in this form. There are two methods for application. The first is to drop the tablets into your radiator and let them get to work. The other option is to dissolve them in some water and then pour that mixture into your radiator. Once in your radiator, the dissolved particles will heat up and seal the cracks. 


Liquid forms of stop sealant are much more common and have a lower viscosity. This makes them move more smoothly through the radiator fluid while also reducing friction. These tend to be made up of additives that help corrode rust and are thicker to fill leaks and reduce dirt and grime.

Top Brands

Bar’s Leaks

First founded in California in 1947 by Fred Barton, Bar’s Leaks now calls Holly, Mich., it's home. It makes high-quality stop-leak products for vehicles, including the Bar’s Leaks Liquid Aluminum Stop Leak.


Headquartered in Chicago, Ill., Prestone makes antifreeze, coolants, brake fluids, and power steering products. It’s been in business since 1927 and sells the Prestone Radiator Sealer Stop.


Another recognizable name in automobile parts is ATP. The company started at a Chicago plant in 1955 and it sells transmission parts, filter kits, repair kits, cables, flywheel and ring gears, cabin air filters, chemicals, and much more for your vehicle. The ATP AT-205 Re-Seal Stops Leak is one of its most purchased items.

Radiator Stop Leak Pricing

  • $5 to $35: With radiator stop leak fluid, you mainly pay for the size and capacity of the container. Within this price range, you’ll find anything around 12 ounces and below.
  • $35 and up: This price range has larger bottles and containers that offer more solution for more applications. It may be best to buy it in bulk to make sure you have extra just in case you run out when you need it most.

Key Features

Ease of Application 

Look for a stop leak that matches your knowledge and skill level. Some require you to flush the entire system before adding the sealant. This may be beyond your skill level or be inconvenient due to the location of the car. If you’re stuck on the side of the road or have little experience with working on your car, then look for one that doesn’t require flushing. 


Additives in radiator stop leak fluids work to remove debris, neutralize acidity, and fill leaks. Some stop leak formulas will include additives that protect your system from corrosion and rust. These are a good option if your car is older as they will help preserve your car and run smoother. Other additives help to make the stop leak more durable so that it expands and contracts with the changing temperature of the engine. 


Before choosing the best radiator stop leak, you need to read the label carefully. Not all anti-leak fluids will work for every car. Many are specifically designed for certain models like Honda or Ford or engine types such as diesel or gasoline. You should also confirm that the stop leak formula bonds with the material you’re trying to repair. Some bond with several materials, while others are aluminum, steel, or plastic specific. 

Other Considerations

  • Sealing Ability. Look for the dimensions of the leak that the stop leak can clog. Some formulas are more effective and can seal larger leaks. You need a formula that can seal the size of the hole in your cooling system. Most coolants can effectively seal a leak that measures up to .250mm. Anything bigger, and it’s best to seek out a professional repair.  
  • Antifreeze and Anti-Boil Compatibility. Some stop leaks are not compatible with your antifreeze. These formulas require you to drain the entire system before you add the stop leak. These are tricky to use and can present a risk to your engine. 
  • Frequency of Use. It is important to consider how frequently you need to use it and its effect on your vehicle in the long run. Keep in mind that fluid can be used multiple times before running out. The more leaks your radiator has, the harder it will be to stop them all. 

Best Radiator Stop Leaks Reviews & Recommendations 2021

Best Overall
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.

Best Value
Bar’s Leaks Liquid Aluminum Stop Leak

Bar’s Leak is well-known for its stop leak products, and among the best is its Liquid Aluminum Stop Leak. The compound making up the fluid works great at sealing leaks, and it also conditions the engine with the use of what Bar’s Leaks call Liquid Aluminum. That’s just stepping one. Step two is known as the Xtreme Cool formula. This solution stops overheating and reduces the temperature of the water in the radiator.

Another great feature is how durable and effective this stop leak is. It easily seals leaks in plastic, aluminum, and metal radiators, heater cores, gaskets, and freeze plugs. One other benefit is this stop leak can be added to the cooling system without having to flush out the antifreeze. It can also be used with all types of coolant, including yellow, orange, pink, red, blue, and green silicate-based and non-silicate-based antifreeze.

While the price is a bonus, you may have to buy in bulk to get more efficient results. Another downside is you will have to wait up to 20 minutes with the engine idling to ensure all the cracks are properly sealed. However, as with all of the products mentioned here, it is not a permanent solution and your vehicle will need to be treated again or taken to a mechanic.

Honorable Mention
Blue Magic Radiator and Block Sealer

Permanently repair and seal your radiator and block with this liquid glass and copper-colored radiator stop leak. It’s a liquid formula that’s designed to repair and bond with your radiator materials. It works in heavy-duty applications, industrial, farm, marine, and automotive equipment. To use, pour it directly into your radiator, and it’ll start to work within five minutes. It’s durable enough to withstand up to 2,000 degrees Fahrenheit, making it durable and long-lasting. It doesn’t contain any fibrous or solid material, so it won’t necessarily clog your radiator or cooling system. 

The downside of this stop leak product is that it’s best suited for large-capacity engines. This could make it more than necessary for the repair of a small car radiator. It also doesn’t dry clear, which can be unattractive.

Honorable Mention
Bar's Leaks Radiator Stop Leak Concentrate

Seal the leaks in your radiator with this concentrated formula. It works to seal small leaks in your radiator or heater core. Bar’s Leaks manufactures this product in the United States. One bottle can treat up to 4 gallons. This formula will address drips and leaks that occur from regular wear and tear or aging of your system. 

This concentrate works with all types and colors of antifreeze coolant and/or water. This makes it versatile and easier to use. The concentrated formula means you only need one bottle for a regular-sized cooling system. It will work with most 6-, 8-, and 10-cylinder engines. For 4-cylinder engines, you only need half a bottle. 

Unfortunately, this concentrate isn’t effective at sealing larger leaks or leaks that are located in the seams, bolts, or other areas of the system. It also won’t protect against overheating.

Honorable Mention
GUNK Aluminum Cooling System Stop Leak

This product may be called GUNK, but don't let that fool you. This effective product comes from a family-owned company based in Charlotte, North Carolina. For more than 75 years, the company has produced high-quality automotive products. This aluminum formula stops leaks in your cooling system. Once applied, it will also work to prevent future leaks from happening. 

This formulation contains a rust inhibitor to help preserve and prolong the life of your cooling system. It also begins to work immediately upon application, which is significantly faster than other products on the market. The best part about this product is that there’s no need to flush or drain the system before you apply it. 

Unfortunately, the package for the product is smaller than other options on the market. This means that you’ll need to buy more if you have several leaks or vehicles that you are working on. You also need to check the cleanliness of your system; it needs to be clean so that you can skip draining and flushing.

Honorable Mention
AlumAseal Radiator Stop Leak Powder

This radiator sealant from AlumAseal is one of the original sealant products on the market. You pour the powder into your cooling system and then wait for the formula to go to work. It’s compatible with all types of antifreeze. 

When you use this powder, you not only seal up the current leaks, but you also create a protective barrier that will prevent future leaks and rust. It will also lubricate your water pump to further preserve and extend the life of your cooling system. 

Unfortunately, this powder is only effective at sealing small leaks or pin-sized holes. You also need to be careful, as it has clogged some vehicles’ cooling systems. The powder may not completely mix into the liquid to flow through the entire system, which can cause overheating.

Honorable Mention
Prestone Radiator Complete Care Stop Leak

This patented formula will work quickly to stop leaks that your cooling system has in the radiator, heater core, or water pump. The formula is enhanced with corrosion inhibitors to prevent future corrosion and pitting. This means fewer leaks in the future. 

This product uses a unique formula that contains both natural and synthetic components. It’s effective at sealing leaks that are .020 to 0.5 inches in size. This means it can seal large leaks without interfering with the overall flow of coolant through your radiator. 

Unfortunately, its claims of effectiveness might be a bit overstated. After all, it's not meant specifically as a "stop leak," it's more of a radiator/cooling system treatment. 

Honorable Mention
JB Weld Perm-O-Seal Radiator Stop Leak

JB Weld is one of the most trusted names in radiator stop leak products. It's a specially formulated and finely-ground compound that’s non-toxic and works well in both professional mechanic shops and DIY applications. There are corrosion inhibitors included in the formula to prevent future rust. 

The smaller grind of the powder particles means that it will mix into your cooling system fluid better and get into smaller crevices and leaks. This makes it more effective at sealing the smallest of leaks. The formulation works quickly and will save you money. 

Unfortunately, the powder may also clog your radiator once you’ve poured it in. This could be a result of using too much powder. This can easily happen if you find that the first vial didn’t effectively seal your leak. There is a fine balance between the powder being totally ineffective and clogging your radiator.

Honorable Mention
Liqui Moly Radiator Stop Leak

Liqui Moly's stop leak is for use in passenger vehicles, commercial vehicles, buses, and more. It seals hair cracks as well as tiny leaks caused by impacts and porous soldering points. The product is compatible with antifreeze and coolant water refiners and works on aluminum radiators and cooling water and heating systems, both with and without water filters.

Liqui Moly is made in Germany and works particularly well on European vehicles. It functions as it promises, and it stops leaks as advertised. It's a great product if you need a quick and temporary coolant system leak repair. It isn't uncommon for the product to seal a leak for several months.

There are not many downsides to this product. It does not solve every type of leak. You may need more than one bottle to stop up larger holes, and it's not a permanent fix.

Honorable Mention
Blue Devil Radiator & Block Sealer

Create an airtight seal in your radiator with this permanent repair for your radiator and or block seals. It’s safe and easy to use for the average car owner. You’ll pour it directly into your radiator and let it get to work. It doesn’t contain any fibrous or solid material that could clog your radiator and cause harm. The stop leak will bond together at the weak points in the metal to shore up your entire cooling system. This stop leak stands out because it expands and contracts with the heating and cooling of your engine. 

The downside of this stop leak is that it can be tricky to use. While the directions say to pour it in, you can’t simply dump it all in at once. Pour slowly and gently stream it in so that it doesn’t build up too much and seal up your system in the process. 


  • 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.


Q: How long until I see results from a stop leak product?

This can be a difficult question to answer, and it all depends on the type and manufacturer of the stop leak. Many need at least 10 to 20 minutes to fully take effect, while others need a few hours. The size of the crack or hole also matters; the larger the crack, the more time it will take to fill it properly.

Q: Is a stop leak a permanent fix?

While stop leaks may seem like a permanent solution to your problem, they only last a short amount of time. To truly fix a leak for good, you will need to take your car to a mechanic to get a better understanding of the leak.

Q: Do I need to drain the cooling system after using stop leak?

Generally, most radiator stop leak fluids do not require you to drain the system after applying the solution. However, read the labeling to see what the manufacturer recommends.

Q: What should I do if the sealant doesn’t work?

Remember, not all radiator stop leaks are permanent. If you don’t see an improvement after applying the fluid, you may need to take it to a mechanic for a more in-depth diagnosis.

Q: Are radiator stop leaks safe?

When used correctly in accordance with the manufacturer’s directions, stop leak is a safe product. However, it can cause more damage if misused. Because the product is designed to clog up leaks, it can clog your entire system if used incorrectly. 

Final Thoughts

If you want a reliable and efficient radiator sealant, consider the K-Seal Multi Purpose One Step Permanent Coolant Leak Repair. You can save a bit of money and still get a great stop leak fluid with Bar’s Leaks Liquid Aluminum Stop Leak.