Our top pick for the best radiator stop leaks, ATP AT-205 Re-Seal, is easily the beast sealant we’ve found, able to deal with synthetic and conventional oil engines with ease, while also boosting performance. It’s also formulated to avoid any potential engine clogs with proper use.
However, not all radiators are the same, which is why we included a variety of stop leak products in our top picks, including tablets and liquids designed to be used with antifreeze. Take a look at the full list to find the best option for your vehicle!
Top 5 Best Radiator Stop Leaks
#1 ATP AT-205 Re-Seal Radiator Stop Leak
Award: TOP PICK
WHY WE LIKE IT: ATP’s stop leak is suitable for many purposes and won’t damage your system! It also works with multiple types of oils for easy applications.
- Restores rubber and stops leaks
- No petroleum distillates
- Treats up to 6 quart capacity
- Not suitable for antifreeze
- One bottle may not be enough
Simply the best oil stop leak we’ve found, ATP’s Re-Seal is the perfect choice for car owners who need a single application to help restore their radiator performance. It’s designed to work with both conventional and synthetic oils, as well as gear oils, hydraulic oils, and power steering fluids. This makes it far more versatile than most stop leaks, able to treat systems outside of just your radiator.
A little also goes a long way for this bottle, which is only 8 ounces but can treat systems with a 6 quart capacity. It can also help make rubber seals and gaskets last longer! Plus, it avoids any petroleum distillates, which makes this solution less corrosive and ideal for more sensitive engines – the best radiator stop leak for BMWs, for example. When working on your car, the best car dolly comes in handy for moving it where you need it.
#2 K-Seal Pour & Go Leak Repair
Award: HONORABLE MENTION
WHY WE LIKE IT: It’s super easy to use and fit for all types of antifreeze, with the ability to help seal leaks throughout many different types of radiator systems. You can finish the project in minutes and be on your way with this solution!
- Best antifreeze-ready
- Seal leaks up to 1/32 inches
- Pour and go formula
- No suitable for oil-based systems
If you don’t have an oil-based radiator, you need a stop leak that works with antifreeze – and this K-Seal is the best option on the market for these systems. No draining or flushing is required: You just pour it in the radiator or overflow and run your engine. The formula works with all types of antifreeze, and can seal leaks up to 1/32 of an inch when cured.
The formula is suitable for radiators, freeze plugs, heater cores, and water pump casing, and is the best plastic radiator stop leak …a useful feature if you aren’t sure where leaks are coming from. The stop leak also provides advice on avoiding over-pressurizing and other problems that might prevent it from working. The K-Seal ST5501 Pour and Go Leak Repair is a US bestseller and has sold over 8 million bottles worldwide, so be sure to check out this popular sealer the next time you have a leak in your radiator. The best car jack can help when working on your vehicle.
#3 ACDelco Cooling System Seal Tabs Radiator Stop Leak
Award: BEST TABLET
WHY WE LIKE IT: These tablets are very easy to store and use whenever needed, making them a great preparation buy if you want to deal with future problems. They’re also non-toxic and can be used with all types of vehicles, specifically antifreeze systems.
- Easiest to use
- Can drive normally after adding
- Dissolve over time
- Takes some time to work
- Requires radiator flush after using
- Not designed for oil-based systems
Another potential stop leak solution comes in tablet forms. Tablets are a great fit for long-term storage, like auto supply kits for a fleet of vehicles. They’re designed to be used in a wide variety of engines (including smaller engines with coolant radiators) and can be quickly administered by crumbling the tablet up and adding it to the coolant tank.
The tablets are also suitable for all DEX-COOL antifreeze types, the “orange” antifreeze that’s required for aluminum radiators. But be sure to flush your radiator soon after using to make sure that no clogs occur due to tablet materials! Since you are already working on the radiator, now might be a good time to upgrade to the best car brakes too.
#4 Bar’s Leaks 1040 6-Pack
Award: BEST BUDGET
WHY WE LIKE IT: It’s an affordable 6-pick of high quality stop leak that’s designed for long-term treatments. It also works with all types of oil as well as gas or diesel engines!
- Excellent rear main sealant
- Easy pour bottle design
- Versatile for all kinds of oil leaks
- Needs a few days to work
Looking for a larger quantity of stop leak for professional purposes or multiple treatments? This Bar’s leak 6-pack is a great buy, highly affordable with an easy-pour bottle design. The formula is meant for rear mains in particular, but it also works with time covers, cam seals, and other leaks in oil-based systems.
One bottle is designed to treat four to 6 quarts of oil. Pour it in and start driving – but give it time to work. It’s made to cycle through your engine for the next hundred miles or so to help seal leaks as fully as possible. It may be the best stop leak for semi trucks thanks to these long-term qualities. If you’re interested in more Bar’s Leaks products, there’s the 1109 Block Seal Liquid Copper and Radiator Stop Leak, which is strong enough to repair larger leaks of all types, like heater leaks, freeze plug leaks, blown head gaskets. etc. Also, another good option if you’re on a budget is the J-B Perm-O-Seal Weld Radiator Stop Leak, which is a durable fast fix for sealing big and small leaks. This might be a good time to install the best car battery as well.
#5 Bar’s Leaks Liquid Aluminum Stop Leak
Award: BEST QUALITY
WHY WE LIKE IT: This stop leak works with all types of antifreeze systems and can be re-applied if necessary to deal with more serious leaks. It’s the stop leak you need if you want the very best results for your cooling system.
- Two-part formula for extra protection
- Works in under 20 minutes
- Treats multiple parts of coolant system
- Not designed for oil systems
A larger 16.9-ounce bottle for the biggest systems, this stop leak uses a combination of two different formulas to help seal leaks first, then condition the system to help prevent future heater core leaks and coolant leaks. It works with absolutely all types and colors of antifreeze, including silicate and non-silicate based versions.
The formula also helps seal freeze plug leaks, lower water temperature, and generally creates safer conditions for your radiator. It also works very quickly, in 20 minutes at the most, and can be re-applied if necessary without worrying about clogs.
How We Decided
One of things we looked for most with stop leaks was how compatible they were with the average engine – and how easy they were to apply quickly when you notice a leak. That means we focused on products designed to work with a wide variety of antifreeze or engine oils, depending on the type of radiator in question (we chose products for both categories). Stop leaks that are made to work with many different liquids also tend to be pour-and-go versions, which the average consumer really needs when buying a sealant like this.
We also looked for stop leaks that specifically help rejuvenate more vulnerable parts of your radiator system as they do their work. That means helping to seal faded rubber and plastic gaskets or hoses so that additional leaks are less likely to form while you search for a more permanent solution. However, we tried to stay away from products with petroleum distillates that may clog modern engines or cause additional problems.
We also awarded more points for stop leaks that come in easy-pour bottles or that allow you to quickly buy in bulk for professional needs. While not as important as some factors, we also liked to see stop leaks that treat larger capacities with less – our top pick, for example, can treat 6 quarts of oil with only 8 ounces.
Best Radiator Stop Leaks Buying Guide
The Most Important Features to Consider
- Ease of Use
Don’t worry about how to use radiator stop leak formulas – pick one that’s easy to apply! We prefer today’s liquid stop leaks that are designed to be quickly measured and poured right into the coolant tank. Those that don’t require any careful mixing, draining or flushing are especially simple to apply when you have a problem. Also note that the shape of the container does play a part in this: Narrow containers designed for single use are simply easier to pour and measure than a half-gallon container. - Compatibility
What is your stop leak designed to do, specifically? Stop leaks can vary a little in their purpose. Some are specifically designed for rear main leaks, for example, while others are made for more general purposes. Also check if your stop leak is designed to be used in coolant or oil, and make sure that this matches the type of radiator that you have. - Safety
Look for models that don’t have petroleum distillates if possible – these stop leaks are more likely to cause clogs and other problems, especially in more sensitive modern engines. Look for products that are rated for all kinds of oils or antifreezes so you don’t have to mix and match. Don’t buy off-brand stop leaks! Your engine health could be at risk without a high quality product like our top picks. - Additional Benefits
Good stop leaks can also help restore older rubber seals and gaskets throughout your engine. This can help make it less likely that more leaks will develop in the future, and it’s a sign that the brand is confident in the abilities of their product.
Radiator Stop Leaks FAQs
Do radiator sealants really work?
These sealants are chemicals that are designed to bond to the internal surfaces in your radiator: They react with trace amounts of oxygen found near leaks and connect with the surrounding metal, essentially patching it from the inside. So yes, these products really do work - the big question is how long they work. Most stop leak products tend to promise “permanent” seals that get your radiator back to new. This is rarely true: A leaky radiator is likely to develop more leaks in the future and should be taken to a mechanic for a closer look, even if the stop leak temporarily fixes your problem. It’s not a good idea to keep adding stop leaks, because they can overwhelm your engine with repeated use.
Can a stop leak clog cooling systems?
No. Early stop leak products used plant matter and had a higher chance of clogging your radiator. Today’s versions are a smoother liquid that are less likely to do any damage to your system. However, always closely follow the instructions for the stop leak you are using, especially any instruction about how much to use, what temperatures to use it in, and how to treat your engine after use. Improper conditions could potentially cause trouble.
Will a radiator stop leak harm my engine in any other way?
They shouldn’t. High quality stop leaks are designed to be safe for all engine components. However, low quality versions used improperly are likely to damage your engine, so it’s important to get a high quality product and follow instructions very carefully. The type of engine may also matter - some engine designs simply aren’t as good for stop leaks, in which case an expert opinion might be helpful to learn more. Some people worry if a radiator stop leak can cause overheating, but this is rare for all high-quality formulas, some of which can even help cool your radiator.
Can You Put Radiator Stop Leak Straight into the Reservoir?
This usually depends on the type of stop leak. Generally most consumer-facing stop leak products these days are designed to be poured or placed directly into the coolant tank. Some are designed to be mixed with your engine oil instead - since certain radiators use oil instead of coolant.
How Fast Does a Radiator Stop Leak Work?
This is another part where it’s important to look at the instructions. Some stop leaks are designed to work more or less immediately. Others are designed to work more slowly to help restore older radiators over the next 100 miles or so. It also depends on the damage your radiator has already suffered. Stop leaks are designed to help shore up wear and tear, or fix pinhole leaks caused by that wear. They aren’t really made for large damage or cracks, and in the best case it may take them longer to patch these problems.
How do you get rid of the stop leak in a cooling system?
Some stop leaks (for example, tablet versions) shouldn’t be circulated forever in a coolant system. Other versions may use compounds that can clog a system over time if it isn’t removed. The only way to fully get rid of stop leaks like these is to flush out the coolant system. If you don’t have the tools or experience for this, a local auto mechanic can easily do it.
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