Fin Bond failure: Old Solder Gone Bad

Welcome to fin bond failure. While it’s not the most common cause of problems for radiators, it is a problem nonetheless.

When it comes right down to it, radiators are simple machines. Hot coolant is pumped into them from the engine, the coolant is then pushed through a series of parallel tubes, either through pressurization or baffles. As the coolant makes contact with the tubes of the radiator, the heat is then conducted to the fins, which are bonded to the outside of the tubes. As the heat transfers to the fins, the cooling fan blows the heat away from the car. Combined with the exhaust system, the theory is that the vehicle’s internal combustion engine should stay within its operating parameters.

Well, that’s the simple theory, at least. The reality is that it’s a bit more complicated than that, and we’ll look more into the real functionality of the complete cooling system in future posts. This post, however, is about the fins of the radiator.

Just like the cooling system itself, fins aren’t quite as simple as the explanation above implies. They also serve a second very important purpose. As you know, the primary purpose of the radiator is to accept extremely hot coolant and, after going through the process as described above, send it back to the engine at a much lower temperature to pick up more heat for the radiator to transfer. The heat within the tubes is transferred to the fins through convection and the cooling fan or windspeed blow the heat off of the fins. The cycle continues for as long as the engine is running at expected temperature (and above).


It’s this cycle of heating and cooling / expanding and contracting that occasionally causes the solder holding the fins to the tubes to melt away and the fins lose adhesion (see the accompanying graphic). This will generally cause the cooling system to get even hotter, and therefore create even more fin bond failure. There is no repairing of a radiator core that has fin bond failure. The only solutions are to recore the radiator or replace the radiator entirely.


Even in the rare case that fin bond failure doesn’t cause excessive overheating, the problem is still terminal and that’s because of the fins’ secondary purpose. When a radiator (or any matter) gets hot, it expands in size; when it gets cool, it contracts. Due to the extreme fluctuations in radiator temperatures, this process is more pronounced than in nearly anything else. The only reason that radiator tubes don’t explode while going through the heat cycle is because the fins are there to resist the expansion. Take away the fins and what do you get? A leaking radiator. In any case, you need to talk to Cap-A Radiator. Call us at 516-293-9026 or come in to 994 Fulton Street (Route 109) in Farmingdale (Long Island, NY) to learn about your options.

Classic Radiator. Please call us at 516-293-2175 to learn about your options.



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Classic Radiator

Classic Heaters 

994 Fulton Street

Farmingdale, NY 11735

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