Hey guys I'm trying to achieve that clean cafe racer look on my 1991 BMW K100LT ABS and I was just wondering if the coolant reserve tank is actually necessary if you ensure your radiator is always topped up?

Your coolant expands and contracts as the engine heats up and cools down.  If not for the expansion tank/reservoir, you would be springing leaks all over the cooling system every time you use it.

Point-Seven-five wrote:Your coolant expands and contracts as the engine heats up and cools down.  If not for the expansion tank/reservoir, you would be springing leaks all over the cooling system every time you use it.

Ok another question then, when I bought the bike I didn't check the levels but I just have earlier today and found there is basically no coolant in the reserve tank (way below the min mark), does that mean this was a bad thing for my bike when the previous owner was driving it? Also can I make my own one to relocate somewhere?

Yes, its a bad thing. Top up the tank with coolant to the minimum mark at least.

Yes, you can use any other design as an overflow tank. Some people have used ex-waterbottles. Some have welded up their own designs. Search the forum for ideas.

DBRMN wrote:. . . there is basically no coolant in the reserve tank (way below the min mark), does that mean this was a bad thing for my bike when the previous owner was driving it? Also can I make my own one to relocate somewhere?

I missed rbm's post during my fervor of concentration while writing this prose masterpiece. He covered it all. Reading mine is just for those with nothing else to do.

"No coolant" is different than some coolant.

It might mean that the previous owner didn't bother checking coolant level, never flushed the system, nor changed the coolant. It could have been hard on the engine if the cooling fan was defective and the bike was ridden extensively while overheating. Coolant might have been lost from boil-over. Regardless, all components of the system should be checked for proper operation; the cooling system should be flushed and the coolant renewed before extended operation.

You can make your own coolant reservoir. The inlet of the hose in the tank should be at or near the tank's floor. Look in custom K-bike threads and you'll see some innovative work. This K1 conversion is nicely done. This photo shows the bike before it got its KOSO speedo. The arrow shows the tank.

If the system has coolant missing, it is likely that the previous owner was rather ambivalent about servicing the cooling system.   This often results in the use of tap water, or even worse well water that loads up the cooling system with a mineral coating. 

That coating causes overheating and boilover which further coats the system with deposits. 

As long as you are working on the bike it would be very advisable to flush all the crap out of the system, especially the radiator.  This will insure that your cooling is optimized and the chances of that stinky antifreeze getting all over are minimized.