What is the Best Radiator Stop Leak in 2024?

If you are in a hurry and just want to find out what the best radiator stop leak is, then we recommend Bar’s Leaks 1186 Liquid Aluminum Stop Leak as the best one.

Did you find a pool of watery liquid under your parked car?

Or maybe the engine temperature warning light came on while you were driving?

These are clear symptoms of a radiator leak. And if left untreated, a leaky cooling system will cause overheating and result in serious (and very expensive) engine damage.

A complete radiator replacement, however, is expensive.

Fortunately, there’s a more economical solution: radiator stop leak sealant.

Read our radiator stop leak review to find the best cooling system sealant to help you fix that leakage quickly and effortlessly. And save a good amount of money in the process.

Here are the best radiator stop leaks we recommend:

why you should trust us

All of our reviews are based on extensive market research, expert input, and practical experience. We invest time and resources to analyze the products. This way, we can offer you genuine, accurate guides to help you get the best pick for your needs.

Do I Need a Radiator Stop Leak?

The engine temperature warning light

If the engine temperature warning light came on, then you likely have a problem with your vehicle cooling system. And you absolutely have to fix it as soon as possible!

The radiator is a component of your vehicle that is used for cooling the engine.

In most vehicles, cooling is performed by circulating a liquid – aptly named engine coolant – through the engine block (where it absorbs the heat) and then through the radiator (where the heat is dissipated).

A car radiator

Cooling is necessary for practically all engines, as part of the motor work will always be converted into heat. It is also critically important since without proper cooling the engine would soon overheat and breakdown.

If the engine temperature warning light came on but there doesn’t seem to be any evident leak, then maybe you’re in luck and you just have a rusty or clogged-up cooling system. You can simply clean it with a radiator flush and everything should be back to normal.

If, on the other hand, you find a pool of watery (not oily) liquid under your parked vehicle, then you have a proper leak, and you need to take a different course of action.

Having a mechanic manually repair the leak is usually very expensive. And replacing the radiator completely is even more expensive.

Radiator stop leaks (aka radiator sealants) represent a very cheap and effective solution to repair minor cracks in the cooling system that may be causing the leakage. And it’s what your mechanic would probably use anyway.

What is Radiator Stop Leak and How does It Work?

A radiator stop leak is a chemical product able to form a seal on the cooling system cracks and thus stop the leak.

Radiator stop leak sealants suffered a bad reputation in the past as the early versions would form a gummy layer that would seal the leak but also sometime clog up the cooling system.

Additionally, these products were often temporary solutions that you had to top-up each time you flushed your radiator fluid.

Fortunately, things have changed over the years and products improved significantly. The best stop leaks today will be able to seal the leaks in your radiator more permanently without clogging or damaging the cooling system in any way.

Modern stop leak car sealants are made from ceramic or metallic fibers. As they flow through the system they embed in any gaps because of the sudden change in pressure. As more fibers get attached, they build a strong mesh which will then harden and cure with the heat of the engine, permanently sealing the leak.

Types of Radiator Stop Leak

There are many types of cooling system stop leaks.

Some of them boast aluminum (like our top pick), copper, or even almond particles in their formulae. However, this is really just marketing. Their exact composition, for obvious reasons, is a trade secret.

Similarly, there are various packages and consistencies they come in, mainly liquid and tablet form. Liquid is the most common and tends to be the most effective, since it allows for the presence of useful additives that help corrode rust and reduce dirt and grime in the system.

How to Use Radiator Stop Leak

You may think this is as simple as pouring it in. And you’d be mostly right. However, there are some precautions to take before and after pouring the sealant in the radiator.

Radiator stop leak being poured

Always carefully read and follow the instructions of the specific product you’re using. Here’s the general gist, however.

  1. First of all, always start with a cool engine, so that the radiator cap can be safely removed.
  2. Pour/drop the sealant into the radiator. Use the recommended amount. Unless the cooling system is visibly dirty or clogged, there’s generally no need to flush it before using the sealant.
  3. Add radiator fluid on top, so as to fill the radiator and overflow tank to proper levels. Then, reinstall the radiator cap.
  4. Start the car and let it run for about 15 minutes (or the amount indicated on the sealant instructions), allowing the engine to warm up. This will circulate the sealant through the cooling system and help to plug any crack and pinhole.
  5. Allow the car to sit overnight so that the radiator sealant can set up properly. Then you can drive as usual.

To see if you’ve successfully stopped a leak, you can leave a piece of cardboard under the car where the fluid used to leak. Check the cardboard in the following days to see if any leakage remains.

How to Choose the Best Radiator Stop Leak

When selecting the best radiator sealant for your needs, here are a couple of things you should consider.


First of all, read the label carefully. Not all anti-leak fluids will work for every vehicle. Some are specifically designed for larger vehicles like trucks and RVs.

Also, see if the sealant has particular requirements regarding the coolant type and antifreeze.

The products we recommend are actually compatible with most vehicles on the market and can be mixed with all types of coolants and antifreeze. However, make sure to double-check the fluid before buying it to make sure you don’t pour an ill-suited stop leak into your vehicle.

Ease of Use

Some car radiator sealants require you to flush out your cooling system before use. Others require you to flush it after. Others, still, can be added straight to the system, mixed with the existing fluids, and left there to continue to protect the components.

These last ones are our favorite type since they are the easiest to use and have an ongoing protective action that makes them doubly effective.

Anyway, do check the usage instructions before making a purchase, in order to select the stop leak most appropriate for your needs and skills.

Leak Size

If the leak is fairly small then the radiator stop sealant will likely be able to fix the problem and prevent further issues. If, on the other hand, the leak is on the larger side, you’re either going to need to apply the sealant multiple times or seek professional help.

These are the Best Radiator Stop Leaks

Alright, here we go. In our experience and according to our in-depth review, these are the absolute best radiator stop leaks on the market.

Best Overall: Bar’s Leaks 1186 Liquid Aluminum Stop Leak

Bar’s Leak is very well known for its stop leak products, and Liquid Aluminum Stop Leak is definitely among the best of its radiator sealant lineup. Its Liquid Aluminum formula not only seals leaks, but also conditions the system.

The best radiator stop leak: Bar’s Leaks 1186 Liquid Aluminum Stop Leak

The product also contains Xtreme Cool, a proprietary additive which stops overheating and significantly decreases water temperature.

The combination of these two elements is able to safely and easily seal leaks in plastic, aluminum and metal radiators, heater cores, gaskets and freeze plugs.

Another great bonus of this stop leak is its ease of use and universal compatibility. It can be added to the cooling system without having to flush out it before or after. 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, as well as water.

Leaks are sealed within 20 minutes but more often than not they are fixed within minutes!

Best for Larger Engines: K-SEAL HD Coolant Leak Repair

K-Seal HD is a great sealant product able to fix most leaks in water-cooled engines, designed specifically for HGVs, tractors, heavy plant and other commercial vehicles.

It is compatible with all coolant system additives and kinds of antifreeze, and doesn’t require a system flush before or after using it. So you can apply it safely and easily, without worrying about compatibility issues.

K-Seal HD’s scientifically-tested formula, containing ceramic microfibers and copper particles, is designed to target holes and cracks and permanently seal them without damaging your engine. It is a trusted product and used by many professionals in the industry.

One final note of warning though: because of the strength of the product, you should be extra careful about respecting the recommended dosage and not exceeding it, as you could incur in clogging problems.

Honorable Mention: ATP AT-205 Re-Seal Stops Leaks

ATP is a well-known name in the world of automotive. And its AT-205 Re-Seal is a fast, effective, and safe stop-leak for all rubber seals and gaskets.

The exclusive plasticiser formula restores and rejuvenates worn or dried out seals and is not harmful to internal bearings and components. It does not contain petroleum distillates and will not over-swell or breakdown seals.

ATP AT-205 Re-Seal is a solid product for minor hydraulic system leaks. And what deserves it an honorable mention in our review is the fact that it actually backs up as a universal stop leak: it can be used for all rubber seals and gaskets in engines, transmissions, power steering, differentials and hydraulic systems. It is compatible with all conventional and synthetic oils, ATF, gear oils, power steering hydraulic fluids.

It is the Swiss Army knife of stop leaks and we strongly recommend carrying a bottle or two in your vehicle trunk should any leaking issue arise while you’re on the road.

FAQs about Radiator Stop Leak

How long does it take to see results from a stop leak product?

The precise time you’ll have to wait for the sealant to take effect will be specified on the product label. Generally, most stop leaks will be able to seal the leak within minutes from the application, but they might require a longer sitting period for the sealant to set in completely.

Is stop leak a permanent fix?

It really depends on the severity of the leak. For minor leaks, a good sealant like the ones we recommend can actually be a permanent fix. For larger cracks, however, you may need to apply the sealant multiple consecutive times or just seek professional help.

How much stop leak should I use?

Read and carefully follow the instructions on the product label! Each product varies and will say on the packaging how much radiator stop leak sealant is necessary to fix the leak. Dosage may also depend on your vehicle size and the capacity of the radiator.
Some leaks will need several applications to form a strong enough seal. So it’s always best to have more product than expected, as you never know when you’ll need to quickly repair your car from a leak.

Is radiator stop leak bad for your car?

Overuse of stop leak is generally the number one mistake most people make. It is wasteful and dangerous, and precisely why some people give a bad rep to radiator sealants, as they probably suffered a clog from excessive dosage.
When applied properly and in the recommended dosage, a good radiator sealant is absolutely harmless for the vehicle. It will fix the leaking issues and actually protect the cooling system from the inside.

Can radiator stop leak cause overheating?

A leaky cooling system actually causes overheating. Solution: use a stop leak product.
Using a poor stop leak product, or overdosing it may cause a clogged cooling system, which, in turn, may cause overheating. Lesson: use a good stop leak product in the recommended dosage.

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

Generally, most radiator stop leak fluids do not require you to drain the system after applying the solution. However, as we said again and again, you should carefully read the label to see what the manufacturer recommends and apply it to the letter to have the maximum effectiveness from the sealant.

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

If you don’t see an improvement after applying a radiator sealant, the leak may be too severe to fix with such a product. In this case, you may need to take your vehicle to a professional for a more in-depth analysis of the problem.


A good cooling system stop leak can save you a lot of stress and quite a couple of bucks when dealing with a leaky radiator.

The best radiator stop leak from our review is Bar’s Leaks 1186 Liquid Aluminum Stop Leak. It is the most effective, compatible, and easy-to-use stop leak product around, and it has proved its worth countless times.

Buy it now and be done with those leaks for good.

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