TO TELL YOU THE TRUTH I HAVE NEVER MEASURE HEAD BOLT STRETCH, BUT BY GOING HOW ROD BOLTS STRETCH, WHICH IS MINIMUL, I WOULD GUESS MAYBE .005 THOU, .150 IS A BUNCH, OF STRETCH EVEN THOUGH THE ARE KINDA LONG , I WILL TAKE A LOOK LATER TODAY AT A NEW BOLT AND A USED ONE AND SEE IF I CAN MEASURE ONE AND GET BACK TO YOU ON THIS ONE , HOWEVER IT WOULD ONLY BE AN APPROXIMATE READING AS YOU KNOW THE BOLT WOULD NOW BE IN A RELAXED STATE , JERRY