Project S2000 - Self-Bleeding Expansion Tank - Modified Magazine

The conventional S2000 cooling system has a vertically mounted radiator with the cap located at the high point in the system (on top of the rad). As the coolant warms up to operating temperature, it expands, causing pressure to build. Once pressure reaches the opening point of the cap, it opens allowing the coolant to spill to the overflow. Once the engine is shut down and the system cools, vacuum formed in the cooling system as the coolant shrinks draws coolant back from the overflow tank, and all is as it was. Because the rad cap is at the high point in the system, any air will tend to collect there and be removed when the cap opens, ensuring efficient cooling occurs.

The lay-down radiator in our S2000 project offers a lower center of gravity and increased surface area when compared with a conventional vertically mounted setup. However, it is no longer possible to reach the cap, even if there was one on the top of the rad, and also, it is too low for the system to properly bleed air out. Therefore, a remote-mounted cap is required. The cap must be placed as high as possible to ensure any air in the cooling system is allowed to escape. This is where the expansion tank comes in.

The expansion tank serves multiple functions in our cooling system. First, it locates the rad cap as high as possible and provides a point to fill the system. Next, it provides a means of connecting bleed ports from the engine and radiator to ensure all air in the system is evacuated by itself. Finally, the return line from the expansion tank, which connects to the thermostat housing just in front of the water pump, increases the system's head (the difference in elevation between two points in a body of fluid), ensuring adequate water supply to the pump and preventing cavitation. An expansion tank differs from the conventional overflow type in that it operates as a closed system; the cap should not open during normal operation.

Now it was just a matter of figuring out where the tank should go. Often it's possible to use an off-the-shelf expansion tank, however, if you want maximum capacity or have limited space available, fabricating your own tank may be the best option. On the S2000, the spot just in front of the driver shock tower works out perfectly and even doubles to help shroud the exiting air from the radiator to the hood. Keep in mind that the cold fluid level in the tank will have to be at or above the highest point in the cooling system. To ensure the tank would have enough air space to compensate for expansion without the cap opening, some calculations revealed at 240 degrees F a 6.5L cooling system with a 22-psi cap will need 0.6 liter of air space. The first step was to fabricate the paper template. Time spent to get this just right saves substantial time and money when fabricating in metal. Based on the template with fluid at the cold level, we calculated 1.2 liters of available air space, so we should be good to go.

Cylindrical expansion tanks are often made from 0.065-inch, utility-grade (3003-H14) aluminum, which is easy to shear/bend/weld and is lightweight as well, so we began working with it. Fast-forward to pressure-testing tank version 1.0, and it became clear that 0.065-inch was just not stiff enough for the shape of tank we had built. Even with a bulkhead brace tacked in the middle of the tank, it swelled considerably and would be at risk of failure after multiple expansion/contraction cycles. It was back to the drawing board.

Tank version 2.0 started with 0.100-inch 3003-H14 aluminum. You can also see where internal reinforcement has been added to further reduce swelling when under pressure. Once fully welded together and with the cap assembly welded in place, the tank passed the pressure test without issue. The increased wall thickness did add 1 pound to the total weight of the tank, but it's a necessary evil.

Next, we turned the mounting bosses on the lathe and welded those up. We also drilled bleed and return holes and welded on XRP aluminum weld-on AN fittings. Finally, for the brushed-aluminum look, we went over the tank with one of the 3M red textured pads that look like an SOS pad. The tank fits like it was designed to be there and is a great addition to the engine bay.

When filling the system, the expansion tank is typically only filled about one-third full, and the air space above provides adequate room for the fluid to expand without requiring the cap to open. A high-pressure 22-psi cap also helps to make sure the system stays closed while increasing the boiling point at the same time. Finally, because the system will self-bleed, all you have to do is check the fluid level after the first heat cycle and you're good to go. No more crazy bleeding rituals required.

Next, we turned the mounting bosses on the lathe and welded those up. We also drilled bleed and return holes and welded on XRP aluminum weld-on AN fittings. Finally, for the brushed-aluminum look, we went over the tank with one of the 3M red textured pads that look like an SOS pad. The tank fits like it was designed to be there and is a great addition to the engine bay.

When filling the system, the expansion tank is typically only filled about one-third full, and the air space above provides adequate room for the fluid to expand without requiring the cap to open. A high-pressure 22-psi cap also helps to make sure the system stays closed while increasing the boiling point at the same time. Finally, because the system will self-bleed, all you have to do is check the fluid level after the first heat cycle and you're good to go. No more crazy bleeding rituals required.