Hello ,
my 86' K100RS restoration comes along well.
When I figure out how I will post some pictures.

I want to refill the radiator after everything was apart,
I remember vaguely that one has to " open the thermostat"!?
before a refill.
Do I remember that correctly or am I mixed up.

As always, thanks for your advise.

Marten

No need to do that I just take my time and as it gets towards full squeeze the hose that goes through the crank cover, sort of pump it and that helps get the bubbles out. when its full screw the cap on tight and fill the overflow and after 2-3 runs and some heating and cooling the system draws all it needs from the overflow container then just fill the overflow to the correct level and all should be OK.

I have just done mine for the first time.
1. ON LT removed left hand side pocket and inserts. Lifted back of fuel tank and drew back to one side until filler visible. (No need to disconnect anything)
2. Removed cap and drained coolant via drain plug on pump.
3. Replaced plug with new copper washer to correct torque.
4. Using a long flexible funnel to get to the opening, I filled radiator with BMW anti freeze and de-ionized (could not get distilled water) water (50/50). BMW said tap water OK but I want mine bike to last forever not just to their replacement business model. Just take it slow is right - about 45 mins with a few teaspoons at a time! I then ran motor to heat it to thermostat opening temp - which released a few more bubbles. Replaced cap, tank and side bits.
5. Used basting syringe to empty expansion tank and filled with new fluid.
Done.

Thank you all for your help, I will use this long weekend to get back at the bike .
Marten