Debris in the threads can prevent the bolt from getting the stretch it needs for a good gasket seal due to thread drag. Remember to run a chaser into the block threads, either one designed for the job, or a slightly dull tap. You don't want to enlarge the thread, just clean up what's there.
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!