Today while inspecting some used radiators, I noted that both of them had a rattle noise at the at the thermostat, while shaking the radiator.
Inspection of both (1 K100 & 1 K75) revealed that the slotted square cut O-ring that seals the thermostat into the radiator area seat (not the cover O-ring) was missing entirely on the K100 and all dislodged & wrinkled up, partially trapped under the K75 stat.
I'm wondering if this is a common failure on K bike thermostats? I'm trying to figure out if these fall off in normal use or if consequently both of my radiators had been mis-handled at some time during thermostat installation?
Please report what you have found on yours????

I think what you are seeing is from mishandling. I have two K-bikes and neither have exhibited dislodging of their thermostat seals.

same rob ....on 3 radiators