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Radiator Construction — Part II — Radiator Fins

Updated: Aug 4, 2020

This is the second post in a multi-part series here on and to help our friends and customers learn more about their vehicle’s cooling system. The information contained herein, as well as on all of our blog posts, is derived from decades in the cooling system industry. In this post, we will cover radiator fins.


Today, fins are the most ubiquitous part of a radiator, but that wasn’t always the case. Automobile manufacturers began building tube-and-fin radiators in the 1934 to replace cellular cores. Due to the superior cooling ability of the tubular radiators, the design became popular very quickly.

The fins of the radiator serve a dual purpose. Their primary role is the dissipation of heat. That’s why automobiles have radiators, after all. Basically, the coolant flows from the engine into the radiator. It is then forced through the tubes. The heat that it carries is conducted to the fins and is then blown off the radiator via the cooling fan(s) or windspeed. After passing through the radiator, the coolant goes back into the engine to absorb more heat and bring it back to the radiator.

The secondary purpose of the fins is to prevent the tubes from exploding. It should come as no surprise that this is also an important function, but how do they do that and why would the tubes explode if there weren’t fins? As we said, the tubes carry extremely hot liquids through them, therefore, the tubes get hot and then they cool. Heat causes pressure and the natural desire of the tubes is to expand. Without the fins supporting them, they would expand until they burst. Occasionally, fins deteriorate from external causes such as road salt or salty ocean air, creating a condition known as fin rot. An even more obscure problem is called fin bond failure. When a core is assembled at the factory, the tubes, fins and headers are assembled in a jig, dipped or sprayed with flux and then baked in an oven until the solder on the tubes sweats all the parts together. At times, that solder fails, usually after decades, but sometimes sooner. When solder gets hot enough, it will melt. If the solder holding the fins to the tubes fails, the fins will fall out, the tubes will expand and the core will need to be replaced. We will be discussing both of these topics (fin bond failure and fin deterioration) in greater detail in future posts.

Different Cores for Different Cars

From a distance, all copper radiator tubes appear the same, but that is far from the truth. The biggest difference is that between a standard core and a high efficiency, as described in the first entry of this series. In that post, we also mentioned flat fins vs. serpentine, so let’s take a look at that first. Flat fin cores were the first tubular radiators to be mass-produced, dating back to 1934. A flat fin style core can be identified by the straight fins that run completely across the core and are perpendicular to the tubes. Each fin is punched out with the tube pattern and creates a very strong core. It is still used today in many industrial applications, from lift trucks and front-end loaders to buses, semis and some of the largest mobile and stationary equipment.

While a big improvement over its predecessors, cellular and honeycomb radiators, flat fin radiators were never known for being the most efficient method of cooling. They are also very heavy and a burden on fuel mileage, which is why they are most used in applications that don’t get good mileage anyway or don’t even go on the highways. However, as mentioned earlier, they are strong and durable and that’s important in many applications.

Serpentine Fins

For vehicles that require cooling ability over durability, such as passenger cars and trucks, we prefer the serpentine core. You can easily tell the difference between a serpentine core and a flat fin, even without this graphic.

While flat fins are – for the most part – flat, serpentine core fins look like corrugated cardboard. The advantage to serpentine cores is that their fins have significantly more contact with the tubes than the fins of a flat fin core. A serpentine core with a high efficiency design can have a fin count of up to 22 fins per inch, expressed as FPI, but it can be a double-edged sword. While a higher fin count will absorb more heat than a core with a lower FPI, there still needs to be enough space between the fins to allow the cooling fan(s) to blow air through the radiator to dispel more of the heat that they absorbed. This is the main reason that fin counts usually max out at 22FPI. Most radiator fins have louvers, which add to the radiator’s ability to throw off even more heat. If you’re looking to increase your car’s cooling ability through a more efficient radiator, your best bet is to go to your local cooling system expert with the details of your project and work together to create a custom solution that is right for your ride! No matter what you drive, you don’t want it to run too hot or too cool!

If you are looking to increase the cooling ability of your vehicle, one option to consider is replacing your radiator with an aluminum one. The most obvious difference in fins is the materials that are used to make them. For decades, all radiators (except Corvettes) were made with copper fins, but since the early 1980s, aluminum has more often been the choice of OEM radiator manufacturers. Both materials have their advantages and disadvantages and we will delve deeply into them in Part V of this series.

In the next installment of this series, you’ll learn about headers.

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