Hi everyone!
    I'm trying to track down an overheating problem... maybe. On the first real ride after I got my 1985 K100rs running, it wouldn't hot start. Someone suggested that the FICU is not letting the bike start because it thinks the bike is too hot. I'm pretty sure my temperature sensor is faulty. I'm replacing it. In the meantime, I'm checking out the whole cooling system just in case.
   I drained the system and pulled the radiator today. It had green coolant, which I've been lead to believe is bad. Not aluminum specific, right? I'll refill with the correct stuff after a flush.
   I also cleaned a ton of bugs and grains of sand out of the radiator and straightened a few bent fins.
   I'm checking the thermostat, and it seems to be opening a little hot. The Clymer book says it should open at 176-185F. It's opening at more like 190-195F. Is that a problem? Should I replace it? If yes, any suggestions for an inexpensive replacement thermostat? I haven't even looked at the BMW price. I'm sure it's ridiculous.
  Thanks guys!
   -Jon

My Haynes manual says it opens at 185^oF and should be fully open at 198^oF
I think anywhere around there would be ok.

Pretty much all coolants are suitable for alloy engines these days

It should be marked on the thermostat and probably is 88°C which is 190.4°F. That will be its start to open temp and usually they are fully open at about 5°C or 9°F more.

A search of this forum will disclose some considerable back and forth on the question of thermostats.  What you now have can rather easily be checked with a pot of water and a thermometer.  
I don't think the color of the coolant has anything to do with whether it is compatible with aluminum or not. The coolant I have is definitely green, and it says on the container it is good for aluminum engines.  Now, the BMW dealer may have another story for you, but....

Sounds like your thermostat is working properly.  These engines are designed to run around 200F and even higher when you're stuck in traffic.  In the summer you will sometimes think the engine is on fire.  It's perfectly normal, just get out on the highway and open her up. 

If a previous owner cheaped out and used hard tap water like mine did you might have to do a couple of flushes to get all the crap out so it can run cooler.  Always use distilled water.  I get mine from our dehumidifier.

I just use demineralized water from the supermarket. Its used in steam iron and costs $1/litre. I always mix it up about 25% stronger than recommended. That way I can just top it up with some water once in a blue moon when needed. But my old girl doesn't use much coolant. I've probably used about 2 or 300ml over the last 12 months. I'd be surprised if you found some coolant that wasn't alloy compatible.

there are a few alternative thermostats that work in varying degrees

recently released is a 86c ...high flow version  suitable for Toyota corolla series ..(and many others )

the standard series thermostats for the same series of vehicles is moderately priced ...the high flow version carries a premium price ...but still is way cheaper than the bmw part ... the range includes the 88c version as well *( I believe this is the stock temp rating for many vehicles including the k  )

the real issue is finding the rubber surround gasket that fits the thermostat and the housing ...I have found one that fits almost perfectly ...although the profile is slightly different ,,, just a slight amount of carefulness is required in the installation ....long term feedback is that I find it to work as it should ...and that seal does the job quite well

we have to recognise that high temps in the k bike are not a unknown thing ,,,I find that with ambient temps of around  28c  quit bearable and all is good ...even better when temps are below this ...and I guess many have this as a standard ...for the folks in hotter areas where the frequency of 25 c and above constantly, the extra heating effect is very noticeable ..burnt inner thighs is a common complaint ...

after a recent head gasket issue in my car ...I think I will defiantly be using demineralized water as the additive to the glycol coolant concentrate , that makes the coolant mix that I like .

Guys, genuine BMW premixed coolant (use straight out of the bottle) is GREEN and is not that expensive. Comes in 1.5 litre bottles. Puts the mind at ease.

Ribbid!

Okay, it seems that A) My thermostat is fine. It's starting to open at the rated temp and fully open 5-10 degrees higher. From the Clymer manual I got the impression that it should start to open 10 degrees below the rated temp and be fully open by the rated temp.
  And B) I guess I was wrong about the coolant. In one of the Chris Harris videos he got all hot and bothered because he found green coolant in a bike he was working on. He said coolants are color-coded by their use and that green means it's for an iron block. I should have checked on that before taking it as confirmed.

   Thanks as always for your help!

Yeah, green coolant isn't such a big deal anymore.  Back when these bikes were new the green stuff frequently had the wrong additive mix and could cause problems.  There were a lot of warnings about using it.  Now, it's almost impossible to find an iron block and even the green stuff is compatible with aluminum now.

But old ideas die hard...