Overflow tank/Expansion tank - Do I need it?

[JoHo]

Hey ladies and gentlemen.
I have a maybe somewhat silly question.

I have made custom side panels to my '87 K75C, but it requires my overflow tank to be moved to somewhere. I asked at a facebook forum if anyone knew about a reasonably priced aftermarket expansion tank and where I could buy it. But several guys made a post on how I dont really need the tank. Is this true?
This is what one guy wrote on the forum:

"Just remove it and throw it to the garbage. You don't need it. I am working my motorcycle over a year without overflow tank
Take off overflow tank. Make hot your motorcycle. Very very hot. Open carefully the cap of the radiator and fill it until full. Them close the cap. When it cools it will be perfect!!"

I am taking this to all you experts instead, because I know that on this forum people actually know what they are talking about.

Anyways, if its false and I really need the expansion tank (which I believe I do) do any of you know a "cheap" aftermarket one that I could get? Because I need to relocate it somewhere inside the bike and not on the side, by the side fairing, if that makes sense.

Thanks in advance.

- JoHo

[alabrew]

I don't think that you really do. Older cars didn't have one and it isn't pressurized, it is just an expansion tank.

"Make hot your motorcycle. Very very hot. Open carefully the cap of the radiator and fill it until full."

I don't think this is good advise. I would want to fill the radiator while it is cold as it will hold the most. One you ride and git it hot, it will puke out the excess.

[Laitch]

It's a pressurized system that allows for expansion of coolant into the reservoir and recovery of it from the reservoir by vacuum. Pressure is controllled by a valve in the radiator cap. The system is designed to stabilize temperature. There is a pinhole in the coolant reservoir cap to relieve pressure if the system boils over. The radiator cap pressure valve can also relieve over-pressure if the system fails. In systems without coolant recovery, you needed to be aware of coolant level by removing the radiator cap when the system was cold then replenish it through the radiator. This system is more convenient because you don't need to raise the tank to check the coolant level. In this system, if coolant is needed, it's added to the reservoir from which it is drawn into the engine's system to regulate temperature. When a coolant system like this has been flushed and filled correctly, it is unlikely to ever need topping up if the radiator, its cap and hoses are in good shape.

[Past-my-Prime]

I have seen at least one build -- I believe it was on this forum somewhere -- where someone used a metal 1 liter water bottle and stashed it somewhere just behind the starter. They ran the hose from the radiator cap to the bottom of this bottle, and that allowed expansion of coolant into the bottle when the engine was hot, and withdrawal back into the radiator when cool.

[Martin]

Any container of a suitable size with a suitable lid will do the job. Drill a hole in the lid to accept the over flow hose and a small vent hole if the hose is a tight fit. A bottle that allows you to see the level is better but not essential as long as you check the level. Somebody actually used a baby bottle as an overflow bottle. Ebay, wreckers and motorcycle shops can supply them just find something that fits.
Regards Martin.

[Laitch]

Any container of a suitable size with a suitable lid will do the job. Drill a hole in the lid to accept the over flow hose and a small vent hole if the hose is a tight fit.

The outlet of the hose must always be immersed in the reservoir fluid, preferably near the bottom of the container. Here are some examples, including the baby bottle version.

[alexg]

The expansion tank allows the cooling system to operate without air mixed in it. High performance engines (100 hp/liter, like in the case of our bikes, qualifies) rely on a fully purged cooling system. Thus, the expansion tank is needed. Otherwise there might be hot spots in the engine that while the overall coolant temp is within normal range.

[daveson]

I think the coolant just might be one of the most critical and under valued parts of the system. One of the things I like about bricks is before a ride,  looking at the right side, it only takes seconds to check the fluid levels (gear box and final drive for leaks) Many vehicles went to the graveyard because of levels not regularly checked, for something as minor as a loose hose clamp.

If it needs to move due to lack of space, many modern batteries are smaller in size. If it's to see the sight tube,  maybe a longer one can be used. I always check it for level, colour and bubbles, sometimes I forget or get lazy.

[K1300S]

and even ignoring the health of the engine...if the radiator gets too hot and the cap has to release a little coolant...where will it go if there is no expansion tank?  it will go on the ground in front of the rear tire most likely.  coolant is very slippery...you do not want it going anywhere near a tire.....

[Chaos]

If it's a custom build it probably won't get ridden enough to matter.  But if you're actually going to ride it distances by all means include an overflow tank.