Thank you guys ! I got the o-rings from my local dealer (they had then in a gasket set)
Glenn,
In fact I´m afraid I have a bad inner o-ring. My bike never runs hot, but this weekend it started overboilling in the middle of a hill. When I drained the coolant it was dark brown color, but I´m not sure it was spooge mixed.
Tonight I´ll clean the circuit and replace the coolant completly. If the bike keeps going hot, then I´ll check the inner o-ring.
Nice riding,
Marcão