I had this same problem with a CR80 with an 8mm bolt, it was an AMC mount as well. Have you looked at the mounting surface on the motor mount, I bet you see a lot of pitting or gouging where the case meets the mount. What I have done is machine off the gouged area, which was about 1/16 of an inch and add a steel washer that thickness, no problems any more. Another thing to look at is your bolt, do your threads on your bolt end before it goes thru your mount? I buy longer bolts, so the un-threaded portion goes thru the mount and add a washer or two, and trim off the excess threaded portion.