cooling tank

Hello all !

Today I had to remove the swing arm, so remove the lower hose to the cooling tank.
Cooling tank I call the he white plastic "box" with the UPPER  an LOWER  level marks and the black rubber cover.
The cooling tank receives the steam/vapor/drops, or what ever, via hose from the cap area, when the cooling cap spring (and plunger) opens due high pressure in the system (I suppose) 
The cooling tank has a second, over flow hose, for obvious overflow reasons.

what is the purpose of the cooling tank ?

or: if I don't reinstall the cooling tank, what will be ?


keep and ride safe !

It's an expansion tank,  it's function is to take the excess coolant as the engine heats up and expands. When the engine cools and shrinks it siphons coolant back into the main coolant system to maintain the proper level. 
That's how I understand it anyway. Removing it will result in a loss of coolant, a little at a time. No big deal if you can check it frequently by removing the radiator cap. Although our bikes don't make that easy, with it being under the tank

Strange ANDYC....

I have two notes:
Whenever I drain the cooling fluid from the drain bolt on the pump side, the fluid comes out clear.
I exchange the fluid, say, every two/three years, but without the part which is in the expansion tank.
Today, emptying the expansion tank, the fluid came out not clear from this tank.
Siphoning should keep the fluid somehow equal clear or not clear, since the hose is connected to the lowest part of the expansion tank.

Second note: the path of the fluid from the system to the expansion tank is when the spring/plunger of the cup is open. 
When the engine cools/shrinks, how the fluid will return in the system since the spring keeps the plunger sealing the way back ?

There is two-way valve in the cap.

And for fluid not being as clear as in the main system - maybe your engine doesn't get hot enough for it to be siphoned out and back again?

Yes, it's a two way valve. The vacuum created as the engine cools draws the coolant back in.

Yes, it's exactly like that !
By blowing back from the siphoning hose, with the cap screwed in, you can see bubbles at the carbs heating hose.
 
Yes, it's exactly like that !
I have never seen the temperature gage even at the middle...
Through all these years the fan has switched on twice.

What a pity, without the expansion tank there is plenty of room to add and reach a greasing nipple for the swingarm bearings....

thank you both !!