Full throttle exhaust gas from most engines under load approaches 2K degrees F. I've confirmed that on numerous dyno runs with exhaust gas monitoring equipment in the ports. A valve that operates in that environment for thousands of miles and decades of time isn't likely to be damaged by a little heat from a torch, unless you get outrageous with it!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!