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Water, and engine coolant for that matter, expands when heated. The volume of the water or coolant increases the pressure of a cars radiator system. That coolant has to expand somewhere - ideally, not onto the ground, creating a slick unpredictable surface. We’ve all witnessed the results of a too-hot radiator and it’s after effects. The engine overheats, and in turn, coolant expands in the radiator, spilling over into the radiator overflow tank, but then it leaks out the overflow bottle too because it has expanded too much from overheating.

The good news is that this is all normal stuff unless of course, the engine gets too hot and operates above it’s normal operating range. When engine temperatures stay in their normal operating range, all is well, and the coolant stays within the system and it also stays within the confines of the radiator overflow tank.

If you are in the process of modifying , designing or replacing your car’s cooling system, an often overlooked item that will be needed is a quality radiator overflow tank. A radiator overflow tank, or radiator catch can, as they are sometimes called are mandatory on most race tracks. To be a little more clear, many race cars use what’s called an expansion tank and it’s not quite the same as a radiator overflow tank. It’s kind of inconvenient to have cars puking hot coolant all over the track. Interestingly, this is how the term “puke can” came into being. 


Types of Radiator Overflow Tanks

  • Radiator Expansion Tank or Radiator Surge Tank: These two terms describe the same part and are sometimes referred to as a coolant surge tank. These tanks are used when the highest point of the radiator is lower than the engine. A Radiator expansion tank or radiator surge tank sits higher than the engine and sees full system pressure. It is equipped with a pressure relief cap and serves as the cooling system’s fill point and it also serves as a tank to allow the volume of coolant to expand in the system. In the event of over overheating, it can spill excess coolant out of an exit spout. Many race tracks and sanctioning bodies require this type of tank.
  • Universal Coolant Overflow Tank or Radiator Overflow Bottle:  In many cooling systems, the radiator’s highest point is higher than the engine and the radiator has a pressure relief cap on it’s tank. In these systems, all that is needed is an extra tank to serve as a radiator catch can that will provide the expanding coolant somewhere to go when it warms up, but also returns it to the radiator when it cools back down. 

How A Coolant Recovery Tank Works

A typical radiator overflow bottle is comprised of a tank of varying size with two nipples and one hose connection at the bottom that connects to the radiator via a hose. This hose connects to the coolant recovery port on the radiator, which is positioned just below the radiator cap, but above the cap’s pressure plunger. When the coolant heats up and expands to the point that it’s volume exceeds the size of the radiator, the excess goes out the port to the radiator overflow bottle. When the coolant cools back down, either running or sitting, the decreased volume creates lower pressure in the radiator and It draws the excess coolant back into the radiator. Inside the radiator overflow tank is another port that sits high above the normal bottle level. In the event that the bottle becomes overfull, the rising coolant tops the higher port and runs out on the ground. 

Which Tank is Right for My Application

If your radiator sits lower than the engine (like many factory older corvettes, etc), then you are going to need  a coolant surge tank, or expansion tank. These fully pressurized tanks can be easily identified because they will have a pressure cap neck built onto them. You have to mount these higher than the engine in the system, and you will fill your cooling system from this reservoir. You can actually add an additional catch can onto a coolant surge tank, because many have an overflow nipple built in below the cap. 

If your radiator sits higher than the engine and it already has a fill neck on it, all you really need is an unpressurized universal coolant reservoir or universal coolant overflow tank. 

Sizing a Radiator Overflow Tank

For a universal coolant reservoir or coolant surge tank, the general rule of thumb for proper capacity of expansion is 12% of total coolant volume for draw down and 6% of the total volume for thermal expansion. It’s always best to err on the large size when it comes to capacity. What you do not want happening is coolant running on the ground all the time, or the radiator sucking air back into the cooling system.

Speedway Motors carries a large variety of universal coolant reservoirs and universal coolant overflow tanks. If you need a pressurized tank with a cap, we have multiple choices from our own quality in house brands to great brand names like AFCO, Dewitts and Spike Chassis to name a few. We also carry beautiful, show-quality radiator overflow bottles made from polished stainless steel or polished aluminum. 

Stop coolant loss and prevent air pulling into your cooling system with radiator expansion tanks from Speedway Motors. Radiator surge tanks are great for any radiator and application with easy-to-mount tabs and quality material construction. Available in your choice size, radiator overflow tanks look and perform great on street rods, race cars, restorations, and everything in between. Trust the best in radiator surge tanks from Speedway Motors, in stock and ready to ship today!