Best Radiator Stop Leaks

A radiator leak may seem small at first, but it can quickly hinder performance in other components of your car’s engine. Radiator stop leaks, also known as radiator sealant, fill in cracks and scratches in your radiator to fix minor damage and protect your car from future wear and tear.

Our product review team has researched and reviewed the five best radiator stop leaks on the market to help keep your car running smoothly.

1. Fastest-Acting: Bar’s Leaks Liquid Aluminum Stop Leak
2. Best for Hot Engines: K-Seal ST5501 Pour & Go Leak Repair
3. Best Powder: AlumAseal Radiator Stop Leak Powder
4. Best Tablet: Bar’s Leaks HDC Radiator Stop Leak Tablet
5. Best Upgrade: ATP AT-205 Re-Seal

Not sure what a radiator stop leak does to help your engine? Check out our buyers guide below to learn its essential functions and the different types you can apply.

What Does Radiator Stop Leak Do?

Radiator stop leak is an additive designed to seal leaks in a radiator that forms over time. It uses ceramic fibers and sodium silicate that coagulate to fill small cracks while also preventing future leaks. Although radiator stop leak is not a permanent solution, these additives target common issues that can be maintained until proper repairs are made.

Different Types of Radiator Stop Leaks

There are a few different types of radiator stop leaks, such as powders, tablets, and liquids. Powders are generally applied directly to the radiator. It’s an easy solution, but may be less effective since adding too much can clog the radiator entirely while applying too little can have no effect.

Stop leak tablets can either be added straight into the radiator or diluted with water and poured in. Although more effective than powders, solutions in tablet form do not always have great sealing properties.

Liquid stop leaks are considered the most effective due to their solubility. They are the easiest of the three to apply thanks to their pour-and-go design and usually offer a very reliable seal.

Bar’s Leaks Liquid Aluminum radiator sealant repairs gasket and heater core leaks using its Xtreme Cool formula. It lowers the water temperature in a car’s coolant system to create a safe environment for your radiator to function properly.

This stop leak also blocks freeze plug leaks and is safe to use with both silicate and non-silicate antifreeze of any color. While other stop leaks can take hours to work, the Liquid Aluminum shows results within 20 minutes of use and can be re-applied as necessary without damaging your radiator.

Key Features

• Cost: About $6
• Xtreme Cool formula
• Seals freeze plug leaks
• Repairs gasket leaks
• 16.9-fluid ounce bottle

What Customers Are Saying

Amazon Review Score: 4.3 out of 5 based on over 2,000 ratings

Bar’s Leaks Liquid Aluminum has a great reputation with around 80 percent of reviewers leaving a rating of 4 stars or higher. Many customers highlight the longevity of their radiator repairs done with Liquid Aluminum, claiming a one-time use lasts more than three months. Some of the few complaints note clogging in their radiator after using this product.

K-Seal ST5501 is safe on radiators, water pump casings, freeze plugs, and even heater cores. While some radiator sealers may only repair, this product’s advanced formula actually rejuvenates rubber seals and gaskets to improve their health, too.

One notable feature of the K-Seal ST5501 – it can be applied to a hot or cold engine to save time on repairs, although customers should exercise extreme care around hot engines. No flushing or draining is required with this stop leak since it works with all types of antifreeze.

Key Features

• Cost: About $14
• No draining or flushing required
• Works with all types of antifreeze
• Can repair head gasket leaks
• 8.0-fluid ounce bottle

What Customers Are Saying

Amazon Review Score: 4.2 out of 5 based on over 4,000 ratings

The K-Seal ST5501 is currently Amazon’s number one bestseller for cooling system additives. Although skeptical at first, many customers who leave positive reviews are surprised at how well the K-Seal ST5501 works on their radiator. Many reported having no leaks after letting their car sit for half an hour after application. While there are very few negative reviews, a few of them note that this product did not sufficiently repair a blown head gasket as advertised.

One of the oldest radiator stop leaks on the market, AlumAseal powder has been used in the automotive repair world for over 75 years. It can seal radiator cracks and lubricate water pumping systems to improve cooling performance.

AlumAseal powder works with all varieties of antifreeze. It’s not very effective against large leaks but works well against pinhole-sized holes and breaks. This powder has also been known to cause more clogs when overused compared to other radiator stop leak products.

Key Features

• Cost: About $21
• Safe on metal, plastic, and rubber cooling systems
• Compatible with all types of antifreeze
• 20.0-gram bottle

What Customers Are Saying

Amazon Review Score: 4.4 out of 5 based on over 200 ratings

Reviews from satisfied customers tend to focus on how well the AlumAseal performed for very small radiator leaks. Close to 90 percent of all reviews are 4 stars or higher. A common complaint from negative reviews is that the recommended amount of AlumAseal is insufficient for a proper repair.

Tablet stop leaks take little effort for big results. Bar’s Leaks HDC Radiator Stop Leak tablets can be added directly to a radiator or mixed with warm water before installing for a clean fix within 15 to 20 minutes. Its formula is designed to inhibit rust and corrosion to prevent future cracks from forming, plus it conditions the cooling system to work more effectively.

These tables help control electrolysis by de-electrifying the coolant in the radiator to prevent grounding issues. The price of HDC tablets is very low compared to other liquid and powder radiator stop leaks, which is a plus as well.

Key Features

• Cost: About $3
• Inhibits rust and corrosion
• Helps control electrolysis
• Conditions cooling system
• 15 to 20 minute repair time
• Six tablets per pack

What Customers Are Saying

Amazon Review Score: 4.3 out of 5 based on over 500 ratings

Slightly less than 70 percent of all reviews for the HDC tablets are 5-star ratings. Although not all customers found success with the HDC tablets, most of them praise its fast-acting solution. One common tip left by customers is to break up the tablet using a hammer before adding it to the radiator for better results.

The ATP AT-205 is compatible with both conventional and synthetic oils, including gear oil, power steering fluids, and even hydraulic oil. It can be much more versatile than other sealants since it can treat other system leaks beyond the radiator.

The AT-205 restores rubber and gasket health, too. Its formula also doesn’t contain petroleum distillates, which can break down the inside of the radiator when too many are present. While it’s capable of sufficient repairs, ATP recommends waiting five hours before full results are visible.

Key Features

• Cost: About $21
• Safe on transmissions, hydraulic systems, and power steering
• Restores rubber and gasket health
• No petroleum distillates
• 8.0-ounce bottle

What Customers Are Saying

Amazon Review Score: 4.5 out of 5 based on over 14,000 ratings

As Amazon’s current number one bestseller in Automotive Replacement Transmission & Parts, the ATP AT-205 has a reputation for consistent results. While its overall ratings are high, many of the top-liked reviews are mixed. While some commend the AT-205 as a plasticizer that seals almost all leaks, others criticize it for harming their car’s transmission despite what the product description says.

In our buyers guide to the best radiator stop leaks, our team highlights several products that will effectively seal leaks in your vehicle’s radiator. We did the research so you don’t have to on a number of different types of radiator stop leaks. Due to its solubility, our review team recommends liquid radiator stop leaks over other types of stop leaks.

*Data accurate at time of publication.