The Wunderladen Racing Honda Civic Coolant Expansion Tank!

The factory coolant expansion tank on the 10th generation Honda Civic (and 11th gen Type R) is a fairly drab, opaque plastic box with some hoses on it. When we saw owners having issues with the factory expansion tank splitting apart and leaking at the center seam we knew we had to come up with a bulletproof solution!

As always, before we know where to go, we have to look at where we've been by examining the factory part. The factory tank is made from a cheap fiber reinforced plastic, which is fairly strong but weakens significantly at high temps, ESPECIALLY if your car regularly overheats at the track (a common Type R problem). The opaqueness of the plastic also makes it difficult to accurately read the coolant level, something that's very important for proper function of the cooling system as too much or too little will hurt cooling system performance.

From the outside the expansion tank looks like a simple box, but we cut one apart at the center seam to see what the inside of the tank looks like, and what's inside is far more complex than what you can see from the outside:

The most apparent feature is the tank is divided into 4 chambers. Coolant enters the tank at the top right side of the tank and spills into the first chamber we call this the inlet chamber, there is a passage at the bottom of the separating wall that allows coolant to flow into the next chamber, which we refer to as the degas chamber. The radiator cap is located above the degas chamber. Once again, coolant flows into the third chamber, which we call the primary chamber, through another passage at the bottom of the separating wall. Finally the coolant gets into the fourth and final chamber, called the secondary chamber, by passing through one more passage at the bottom of the separating wall. Once coolant is in the secondary chamber it can reenter the engine through the port at the bottom of the tank.

So why did Honda construct the expansion tank like this? It's all about getting as much air as possible out of the coolant. Air in the cooling system makes the engine run hotter so we want to remove as much air from the cooling system as possible when we bleed the cooling system. The role of an expansion tank is to give a place for air in the cooling system to live until it can be bled out, there is always going to be some amount of air in the system, so ideally we want to move it to the expansion tank and keep it trapped in there so it can't get into the engine.

Air bubbles will always rise through water/coolant. The separating walls that divide the tank into four chambers act as baffles. Each baffle has a passage at the bottom to force coolant to go low in order to get to the next chamber and another passage at the top for air to move to the degas chamber where it will be bled from the system when the radiator cap is removed. This multi baffle system does a great job of removing entrained air from coolant and making sure minimal air gets back into the engine.


So now that we've studied the factory expansion tank, we know what goals we have for our coolant tank:

1. Make it from a material that won't fail under high heat conditions

Our coolant tank is made from entirely aluminum. We chose aluminum because it allows us to make a lightweight tank that can handle high temps, and also do a better job of shedding heat since aluminum has a very good rate of thermal conductivity.


2. Provide a way to read coolant level easily and accurately

Many other aftermarket coolant tanks use a clear plastic tube which is prone to leaking, making it an immediate no for us. Alternatively others use a small round level indicator but all this tell you is if coolant is up to that point or not, it doesn't allow you to see what the actual coolant level is. Not being happy with any off the shelf parts on the market, we ended up machining our own sightglass! This sightglass allows for easy viewing for seeing exactly how much coolant is in the expansion tank. Coolant should not be added above the top of the sightglass, other manufacturers put their small sightglass above this point to allow them to market their tank as being higher capacity, but this leaves inadequate space for air to get separated and trapped at the top of  the tank and more likely for the air to get pulled back into the engine.



3. Include baffling to create good air/coolant separation

We feel the factory expansion tank has a very well design baffling system so we opted to recreate a similar 4 chamber system in our coolant tank to make sure our coolant tank does AT LEAST as good of a job as the factory tank at separating air from the coolant. We did make the passages at the bottom of the separating walls wider to allow for greater flow of coolant from chamber to chamber but other than that it is very similarly laid out to the factory baffling system!



4. Make it fit ALL 10th generation Civics (and 11th gen Type R)

The coolant tank between the non Type R and the Type R is the same basic design with the main difference being different hose sizes that connect to the tank. We were able to meet this goal by using changeable fittings for both the inlet and outlet ports on the tank. We used -10 ORB weld bungs for a leakproof seal on both fittings and -10 ORB is the largest possible fitting we could fit and we went that large to ensure neither the inlet nor the outlet of the tank would be a restriction to the hose that connects to it. No other tank on the market has ports this large!

Note: This does not fit the non Type R 11th gen Civics because Honda changed the cooling system on these cars to use a non pressurized overflow system and does not have a pressurized expansion tank.



5. Ensure great fitment and is aesthetically pleasing

Aesthetics is a bit of a subjective matter but we feel our coolant tank looks right at home in the engine bay and helps tie together the look of the bay instead of sticking out like a sore thumb like the factory tank does.

We were able to ensure robust fitment by utlizing the factory mounting brackets and even replicating the load bearing post on the bottom of the factory coolant tank. We didn't feel that 2 small M6 bolts was sufficient for reliably holding a coolant tank full of coolant in place. The bottom post locates the tank in the rubber bushing like the factory tank and handles bearing the load of the tank and its coolant while the 2 M6 bolts locate the tank and keep it from shifting.


We're absolutely happy with how these turned out and feel it checks all of the boxes with no compromises! As with our other products these are made entirely in the USA, from the manufacturing of each individual piece, to the welding and powder coating, it's all done here! If you're interested in getting one for your Civic be sure to follow us on our Facebook, Instagram, or sign up for our email newsletter for when the product goes live on our site!

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