helped a man with a 340 breakin. fired right up & I held it to 2200-2500. oil psi was 80. water temp climbed to 160 fairly fast. no leaks & sounding good & all of a sudden after about 3-4 minutes it suddenly started to slow down somewhat noticeably & I was manning the throttle & I thought I had with my hand just relaxed/backed off a bit so I moved the throttle up a bit but it stayed the same then 2 seconds later I heard a barely noticeable possibly but not sure metallic "hit" then it died 1 second later. we put a socket on the crank & it is locked solid. No info on clearances. thank you for your time.