After researching manufacturer specifications, industry standards, and aggregated owner feedback, we compared the top stop-leak products designed for cracked cylinder heads. These sealants aim to temporarily repair cracks in engine blocks and heads to stop coolant leaks, but they are not permanent fixes. Our evaluation focuses on performance, durability, ease of use, and safety. Below are our findings for seven real products available on Amazon.
| Photo | Product | Score | Buy |
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K-Seal ST3501 Coolant Leak Repair Top Pick Single-step pour-in formula, works with all coolants |
9.0 | 🛒 Check Price |
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Bar's Leaks 1100 Cylinder Seal Runner Up Ceramic fiber formula, seals cracks and head gaskets |
8.7 | 🛒 Check Price |
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Liqui Moly 3720 Oil Leak Stop Best for Oil Leaks Oil-based formula for cracked heads leaking oil |
8.4 | 🛒 Check Price |
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Steel Seal SS-1 Head Gasket Repair Strongest Seal Two-part chemical reaction, requires water flush |
8.1 | 🛒 Check Price |
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ATP AT-205 Re-Seal Stops Leaks Versatile Sealer Works in oil, coolant, and transmission fluid |
7.8 | 🛒 Check Price |
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BlueDevil Head Gasket Sealer Popular Choice Pour-in formula, requires 50 minutes of engine run time |
7.3 | 🛒 Check Price |
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Gold Eagle 10001 Stop Leak Budget Option Low-cost pour-in, works on small leaks |
7.0 | 🛒 Check Price |
1. K-Seal ST3501 Coolant Leak Repair: Top Pick
K-Seal ST3501 is a popular pour-in stop leak that uses copper particles to seal cracks in cylinder heads and blocks. Based on owner feedback and the spec sheets, it effectively stops coolant leaks in many cases, especially for hairline cracks. The application is straightforward: just shake the bottle, pour it into the radiator or coolant reservoir, and run the engine. Many users report success within 30 minutes of driving.
However, this product is not a permanent repair. The seal can degrade over time, especially under high heat or pressure. It also carries a risk of clogging the heater core if too much is used. We recommend it only as a temporary emergency fix or to buy time before a proper weld or replacement. Skip this if you expect a permanent solution or have a severely cracked head.
- Uses copper particle technology to seal cracks up to 0.025 inches
- Compatible with gasoline and diesel engines
- No need to drain coolant: just pour directly into radiator
Pros: Simple application with no special tools required; Works on a wide range of crack sizes
Cons: May clog heater core if overused
2. Bar's Leaks 1100 Cylinder Seal: Runner Up
Bar’s Leaks 1100 Cylinder Seal uses a ceramic fiber formula that we researched to be effective on cracked heads and head gasket leaks. According to manufacturer specifications, it creates a permanent seal by bonding to metal surfaces. Many owner reviews highlight its ability to stop leaks without clogging the heater core. The application is similar to other pour-in products: add to the radiator and idle the engine.
That said, in our evaluation, the seal’s durability depends on the crack size and engine condition. For larger cracks, a second treatment may be needed. It also may not hold up under extreme driving conditions like towing or racing. Skip this if you have a massive crack or need a guaranteed long-term fix.
- Contains ceramic fibers that bond to metal surfaces
- Designed for cracked heads, blocks, and blown head gaskets
- Non-clogging formula safe for heater cores
Pros: Safe for all cooling system components; Works on both cracks and gasket failures
Cons: May require two treatments for larger cracks
3. Liqui Moly 3720 Oil Leak Stop: Best for Oil Leaks
Liqui Moly 3720 is designed for oil leaks, including those from cracked heads. Based on aggregated owner feedback, it works by swelling rubber seals and gaskets to stop oil seepage. The application is simple: add the bottle to your engine oil. It is not a weld but a chemical conditioner that can reduce or stop minor oil leaks.
However, this product is ineffective for coolant leaks and may not seal larger cracks. It also may not work if the crack is too large or the engine is heavily worn. Skip this if you need a coolant stop leak or have a major oil gusher.
- Specifically formulated for oil leaks in engine blocks and heads
- Swelling agents rejuvenate seals and gaskets
- Add to engine oil, not coolant
Pros: Effective on oil leaks from cracked heads; Compatible with all motor oils
Cons: Not for coolant leaks
4. Steel Seal SS-1 Head Gasket Repair: Strongest Seal
Steel Seal SS-1 is a two-part chemical repair that we researched as a more permanent solution. It requires draining the coolant, flushing the system with water, adding the product, and running the engine for an extended period. Based on owner feedback, it can seal larger cracks and head gasket failures that other products cannot. The resulting seal is reported to be durable under normal driving conditions.
However, the application process is involved and not suitable for a quick fix. It also costs more than typical pour-in sealants. If you are not comfortable with a multi-step procedure, this product may be frustrating. Skip this if you want a simple, fast solution.
- Creates a ceramic-metal composite seal
- Requires draining coolant and flushing with water
- Engine must be run at operating temperature for 50 minutes
Pros: Claims to seal larger cracks and head gasket failures; Permanent repair according to manufacturer
Cons: Complex installation: multiple steps and time-consuming; More expensive than other stop leaks
5. ATP AT-205 Re-Seal Stops Leaks: Versatile Sealer
ATP AT-205 is a versatile stop leak that works by conditioning rubber seals rather than sealing metal cracks. Based on manufacturer specifications, it is intended for leaks from gaskets and seals, not for fixing a cracked head. Some owners have used it on small head cracks with mixed results, but it is not the primary purpose. The ease of use is high: just add to the appropriate fluid.
In our evaluation, this product is not the best choice for a cracked head. It may slow a minor seep but will not stop a significant crack. Skip this if you have a confirmed crack in the metal of the head. It is better suited for leaking gaskets.
- Formulated to rejuvenate rubber seals and gaskets
- Can be added to engine oil, coolant, or transmission fluid
- Safe for all system components
Pros: Multi-purpose: works on various leaks; Easy to use: just pour in
Cons: Not specifically designed for metal cracks; May not seal large cracks in heads
6. BlueDevil Head Gasket Sealer: Popular Choice
BlueDevil Head Gasket Sealer is a popular product that we compared based on aggregated owner feedback. It is designed for head gasket leaks but is also used for cracked heads. The application requires adding the sealer to the radiator and running the engine for 50 minutes. Many users report success initially, but some note that the seal fails after a few months.
This product is best as a temporary fix. It may not hold up under high stress or extreme temperatures. Skip this if you need a reliable long-term repair or have a large crack. It is a gamble for permanent use.
- Seals head gasket leaks and small cracks
- Non-clogging and safe for cooling systems
- One bottle treats most engines
Pros: Widely available and well-known brand; Can seal multiple leak points
Cons: Long run time may be inconvenient; Mixed reviews on long-term durability
7. Gold Eagle 10001 Stop Leak: Budget Option
Gold Eagle 10001 is a budget-friendly stop leak that we researched for minor coolant leaks. It uses polymers to seal small cracks and gaps. The application is simple, but based on owner feedback, it is only effective for very small cracks or pinhole leaks. It may clog the heater core or radiator if overused.
This product is not recommended for a cracked head unless the crack is tiny. It lacks the durability and sealing power of more advanced formulations. Skip this if you have a confirmed crack in the head or block. It is best for minor seepage from gaskets.
- Contains polymer sealants that expand on contact
- Works with all antifreeze types
- Easy pour-in application
Pros: Very affordable; Simple to use
Cons: May clog small passages; Not effective on significant cracks
Frequently Asked Questions
Can a stop leak permanently fix a cracked head?
No, most stop leaks are temporary repairs. They can seal small cracks for weeks or months, but they are not a substitute for welding or replacing the head. Over time, heat cycles and pressure can cause the seal to fail.
Will a stop leak clog my heater core?
Some stop leaks, especially those with large particles, can clog heater cores. Products like Bar’s Leaks 1100 and K-Seal claim to be non-clogging, but there is always a risk. Follow dosage instructions carefully.
How do I apply a stop leak to a cracked head?
Most pour-in products require adding the sealer to the radiator or coolant reservoir while the engine is cool. Then run the engine as directed, usually at idle for 20-60 minutes. Always check the product’s specific instructions.
Can I use a stop leak if I have a cracked head and a blown head gasket?
Yes, many stop leaks are designed for both cracked heads and head gasket leaks. However, if the crack is large or the gasket is severely blown, a stop leak may not be effective. It is best as a temporary measure.
Is it safe to drive after using a stop leak?
Generally yes, but monitor coolant levels and engine temperature. The seal may not hold under hard driving. If the leak returns, stop driving to avoid engine damage. Consider a proper repair as soon as possible.
Our Verdict
After researching and comparing these products, our top pick is K-Seal ST3501 for its ease of use and strong performance on small to medium cracks. The runner up is Bar’s Leaks 1100, which offers a safe non-clogging formula and works on both cracks and gaskets. Both are temporary solutions; for a more permanent fix, consider Steel Seal SS-1 if you are willing to invest time and effort.
