OC without voltage, CANNOT damage my card?? ARE YOU ABSOLUTELY SURE??

changing the clock cannot directly damage your GPU (unlike overvolting), although the extra heat may damage it. just start up a miner, and increase the core clock until it crashes, take your last clock, decrease by 10 mhz, and that  should give you a stable clock. If you're concerned about heat, just put fan to max, and overclock until the heat is higher than 80 C