DIY Coolant overflow / reservoir tank

How it Works

When a liquid is under pressure the boiling point rises. When things are heated they expand. The coolant system is a sealed system, and when the coolant system heats up, this automatically generates pressure, because of this expansion. This is good, to a point. The hotter it gets, the more pressure it builds, and the higher the boiling point of our coolant. We cant let the system get to hot (Not much over 235F) or build to much pressure because, the rubber hoses that the coolant runs through might fail or we might just end up overheating altogether and causing more extreme damage (most likely).

So what happens when the coolant system builds to much pressure?

The cap has a built in pressure valve that lets off the excess pressure. This excess pressure or excess coolant that has now expanded and needs to leave the coolant system needs some place to go other than the ground. This is where the Coolant tank or Coolant expansion reservoir tank comes in. The coolant is captured in this tank.

What happens when the Coolant system cools down?

When the coolant system cools down, the system creates a low pressure area (lower than atmospheric pressure). This void or vacuum in the coolant system draws the coolant back out of the tank and back into the system. (You physicist will say the atmosphere pushes the coolant back into the coolant system.)

Now you see why we just don’t dump the coolant out on the ground. We need it later when the system cools back down, so we have a sufficient amount of coolant when we go back out.

Now that we know how it works, LETS BUILD A LIGHT WEIGHT VERSION.

Materials list

Below is the most common way to build your own coolant reservoir.  There is nothing wrong with doing it this way, but I myself have a problem with it.  When the coolant system does boil over and the reservoir is filled or maxed out.  Where does the coolant go?  The coolant will go all over the place and possibly onto the racing surface This will cause issues for you and the guys behind you.  Not only is there the possibility of leaking onto the track, and shutting it down for a bit. Allowing the coolant to blow past the hose and go where it wants will also make a mess of your bike causing coolant to go everywhere. I feel there is a better way to control coolant overflow.

Coolant reservoir for racing and competition purposes. Hose in the bottle. Simple design
This is the most common way to make a coolant reservoir. A hole is drilled in the cap and pushed into the container until it reaches bottom.  This will then draw coolant from the reservoir and back into the radiator.
Race or Track coolant reservoir DIY
8oz bottle and 3 90deg fittings
Drilled Pilot holes into the cap
Drilled pilot holes in the cap
Enlarged the holes for the fittings
Enlarge the holes just large enough for the threads of the fitting to pass through the holes. I did not spend the extra time tapping the holes for the fittings.
Double fittings in Coolant overflow cap
This is basically how it should look. The coolant in and the coolant boil-over out.
Trimmed barb
Trimmed up one of the barb fittings like so. Leave some matterial, because some of the length will be lost when inserted into the male pipe thread end of the barb / pipe fitting.
Sand down the barb fitting
Trim some more material so the barb fitting will fit into the pipe thread side of the fitting
Lined up assebly of the Coolant reservoir or overflow
The sanded down barb fitting will fit into the male pipe thread end of the Barb / Male pipe threaded end of the fitting. Before putting these items together with epoxy. Sanding the surfaces that will be epoxied will ensure a good mechanical bond. I recommend cleaning them thoroughly with rubbing alcohol. This will ensure all the excess release agent will be cleaned off and the epoxy will have a clean surface to adhere to.
Two part 3M epoxy
I used a two part epoxy by 3M to fuse the two parts together. I found using this was the best method to seal up the nipples.
Mix the epoxy thoroughly
Mix the epoxy until the color is uniform. I only have 10min to work with this stuff before it starts to set up. It takes 24 hours for full cure, and 3 days for maximum cure.
Epoxy on the trimmed barb fitting
I placed just a bead of epoxy onto the end of the trimmed and sanded barb fitting. This end will then be slipped into the pipe threaded end of the other fitting.
Epoxied fitting assembly
I twisted the trimmed fitting into the pipe side. Twisting helps get the epoxy into the joint for a solid seal and bond.

Epoxied fitting for the Coolant reservoir or overflow

Underside epoxying of underside of cap for the Coolant reservoir or overflow.
The epoxy works its way under around and through the hole. A bit more needs to be added on the underside to prevent the fitting from pulling out.
Finished Coolant Reservoir Overflow
This is the finished unit. I placed epoxy onto the pipe threads of the fitting and inserted it into the slightly over-sized hole. I am trying to get the epoxy to seal the area around the fitting
Coolant reservoir vent and overflow inlet.
Attaching a short length of hose to the flair fitting on the inside of the bottle. As long as the end of the hose in the bottle is submerged in coolant, it will draw coolant from the bottom of the bottle, and flow back into the radiator when the coolant system cools down.


Riding motorcycles and wrenching since my preteen years, I have moved from motocross to street bikes. Being a teen back in those days it was tough to get me off of the bike. Now days even though I am very busy being a dad I still have my weekends and go to the track to race and on occasion will do a track day or two.

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