Best Radiator Stop Leak Reviews 2018

Best Radiator Stop Leak- Winner Reviews

Radiator stop leaks are a touchy subject in the automotive world, but there are products out there which can be an effective stop-gap measure to keep your car up and running. If you’re looking to avoid lemons, or even just determine if a stop leak is the right product for you to spend some money on then you’re in the right place. We’ve dug up four effective products and the information you need to make an informed choice. If you’re in a hurry, go directly to our winner: Bar’s 1196-6PK.


Bars 1196-6PK

Best overall: Bar's 1196-6PK

If you lack the knowledge of your radiator beyond knowing that it’s dripping a bit, then use this Bar’s product to take care of the problem until you can afford to see a mechanic.

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Top 5 Radiator Stop Leaks

Rank Name Intended Material
1 Bar’s 1196-6PK Radiator Stop Leak All Check Price
2 Bar’s Leaks 1186 Liquid Aluminum Aluminum Check Price
3 Bar’s Leaks 1109 Liquid Copper Copper Check Price
4 K-Seal ST5501 All Check Price
5 Genuine GM Fluid 3634621 All Check Price

1. Best Overall

Bars 1196-6PK

Bar's 1196-6PK Radiator Stop Leak

Product Rating

Bar’s also offers a fairly generic product which seems to work as well as their more specialized blends. This one is billed for all radiator types and does a good job at handling small leaks and problems which occur over time.

This blend is billed for handling the small damage which occurs over time, and if you catch a slow leak quite early in its development you’ll be able to take care of the problem by using it carefully.

We really only recommend using it if you’re not car savvy and you’re missing an owner’s manual, since a product designed specifically for the material you’re using it on is usually the best bet.

Pros and Cons

  • Works on all radiator types
  • Affects whole coolant system
  • Not a specialized product

2. Best Liquid Copper Radiator Stop Leak

Bars Leaks 1109

Bar's Leaks 1109

Product Rating

Older vehicles often have copper radiators and this fairly reactive metal can cause some complications if you’re not using the right product. This is a great temporary fix for copper radiators and treats the whole coolant system as well.

Keep in mind that this product is primarily designed for pinholes and hairline fractures, anything big enough you’re seeing coolant leaking on the driveway is going to be a bit much for it to handle. It can prevent smaller problems from growing for quite a while, however.

While it’s billed as fixing blocks… don’t trust it for that. Use it as a radiator stop leak, however, and you’re unlikely to run into any trouble.

If you’ve got a slow leaking copper radiator, then give this one a shot. It’s your best bet to prevent further damage until you can make it to the shop.

Pros and Cons

  • Designed for copper radiators
  • Treats whole coolant system
  • Designed for small leaks only

3. Best Liquid Aluminum Stop Leak

Bars Leaks 1186

Bar's Leaks 1186

Product Rating

Bar’s Leaks is another top brand which has been in business for a long time. They make a variety of formulas, many of which are designed for a specific type of radiator.

This stop leak is created specifically for aluminum radiators, so give it a miss if you’re working with the copper variety. This one works for the whole cooling system, giving it an edge up over those which are designed solely for use on your radiator.

It’s no K-Seal, however, and is best used for slow leaks instead of a problem where you’re having to top things off every couple of days.

Give this one a shot if you’ve got an aluminum radiator and a slow leak problem. If you follow the instructions, you’ll be good to go and might be able to stall the fix for awhile.

Pros and Cons

  • Designed for aluminum
  • Treats whole coolant system
  • Not for larger leaks

4. Best For Small Leaks

Genuine GM Fluid 3634621

Genuine GM Fluid 3634621 Cooling System Seal

Product Rating

These pellets are cheap and effective. In fact, GM routinely adds them to all of their manufactured vehicles while they’re still on the assembly line as a preventative against hairline cracks that are bound to occur within the radiator.

While they’re primarily used for aluminum radiators, there’s nothing that won’t interact with a copper option here and it’s a good measure to take. You’ll have to figure out how many liters of coolant are in your system, however, and add a precise amount which can be a bit cumbersome compared to a liquid product.

This isn’t for a serious leak, however, but instead for hairline fractures where you’re having to top off every week or two. It’s a semi-permanent fix but you’ll still want to see a specialist at the first opportunity.

For small leaks, these pellets can work wonders but they’re not really useful for serious problems.

Pros and Cons

  • Cheap
  • Perfect for hairline radiator fractures
  • Can be cumbersome to use

5. Another Great Option

K-Seal ST5501 Multi Purpose

K-Seal ST5501

Product Rating

K-Seal has been in the stop leak business for a long time and their products offer you a good shot at taking care of your radiator. They’re a proprietary liquid blend which is billed as safe for all radiators.

Of all of the manufacturers of stop leak they’re probably the one that you’d want to trust, and their products seem to be good for all parts of the coolant system instead of just the radiator itself.

Of course, they come with the caveat that they’re a bit expensive. They don’t quite reach the price of some of the super-premium brands, like Blue Devil, but they do run at a decent price.

K-Seal is probably the most solid option to go with in order to get a long lasting fix on a serious radiator problem. Remember that even the best is temporary, but this is your best shot if you’ve got a serious problem.

Pros and Cons

  • Works on whole coolant system
  • Trusted brand
  • Expensive

When to Use a Radiator Stop Leak

Before you consider using one of these products, keep the following in mind: chemical solutions to radiator problems are not intended to be a permanent fix, no matter what the bottle says.

In general, you’ll find these products useful primarily for tiny “pin hole” leaks, rather than wanting to use them on anything which can actually be seen with the naked eye. You also want to use the least aggressive product possible for the best results, since stop leak products can cause problems in and of themselves.

It can be hard to tell just how aggressive a product is from the marketing around it, since almost all of them are billed as a permanent solution to the problem, which isn’t really true.

Basically, the more coolant you’re losing over time, the more aggressive of a product you’ll want to use. If you get something too gooey, however, the product can also cause clogging in the radiator which will render everything ineffective and you’ll have to replace the part anyways.

If you’re using a stop leak product, it should only be as a gap until you have the time and money to invest in a new radiator.

How Does a Radiator Stop Leak Work

Rather than mystifying the products, like the manufacturers tend to do, let’s discuss exactly what makes these products work.

Radiator stop leaks work by adding a fiber or other mass to the coolant system. Naturally, these larger particles will tend to be attracted to the lower pressure areas, namely where the leaks are, and clog together to form a gap.

Some products will use a low melting point metal additive as well, forming a temporary weld when the radiator cools down after usage. These can be great for some applications, but it can be hard to tell which is which when you’re just looking at the bottle. Basically: the companies are loathe to say exactly what their product does since it runs the risk of their formula being stolen.

If this sounds kind of dangerous… it can be. Never use more than recommended. You run the risk of fouling your entire coolant system and causing damage to more than just your radiator.

How to Use a Radiator Stop Leak Effectively

Make sure you follow all of the instructions on the bottle in addition to the following general outline in order to make sure you run the best chance of fixing things. Some products will need to be shaken and pellets and powders will have specific directions for how much to add. Follow the amounts to the letter. Too much is definitely not a good thing.

First, you’ll need to let the car cool down and remove the radiator cap. Don’t try to remove it when the car is still hot for obvious reasons, a face full of steam definitely isn’t a good time. Even if it’s cool be careful, slowly crack the pressure rather than just ripping it open. Pressure differentials can do some weird stuff.

Afterwards, you’ll want to add the product that you’ve chosen and allow the engine to warm up to normal running temperature. Keep an eye on the ground under the vehicle to figure out when the leak stops. Once everything stops, let the engine run for a few more minutes and then you can turn the key.

Even if the product doesn’t mention it, we recommend flushing the system while it’s still hot. The last thing you need is excessive amounts of fibers or low melting point metals running around in your radiator. The build-up can cause serious problems throughout the system, and while it’s not guaranteed you’re already looking at the expense of replacing a radiator so it’s probably best to err on the side of caution.

If you’re particularly paranoid, you can flush the whole system twice with water before adding the manufacturer recommended antifreeze back into the system.

From now until your radiator is replaced or repaired you’ll want to keep an eye on the temperature and fluid level. Get things replaced as soon as you can, you might be looking at anywhere from a couple hundred to more than a thousand miles before you have to actually take care of things but remember that a radiator stop leak is not a permanent fix.

How to Pick a Radiator Stop Leak

How to use a radiator stop leak

While there’s a common perception of stop leaks being automotive snake oil, the fact is that some will work in order to let you avoid minor repairs for anywhere from a short time to an extended period.

We’ve done the work of digging through the reviews of this extensive part of the market in order to bring you the best and try to ascertain the situations they work best in. The main thing you’ll want to do is check to make sure the product is compatible with your radiator material.

Those products which are compatible with only one type of radiator may be the best to go with, but it can be hard to tell. This limited capability usually indicates a more precise chemical reaction however.

For the most part, pellets and powders will be a cheaper option. They’ll also work just as well as most fluids, since the solvent matters less once they’re dissolved.

Fluids on the other hand are a bit more foolproof as most are made to fit a specific size of cooling system.


Whatever you choose to go with, you should be able to get back on the road pretty quickly but get that radiator fixed properly as soon as you can. You wouldn’t just tape gauze over a cut that needed stitches, but it might let you get to the doctor without losing too much blood. Pretty much the same principles apply here.




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