Its down to a thread differential, heat will have no affect on it as the threads don't change with a heat cycle?? If you ever strip one of these rocker arms it all becomes apparent. In certain classes of Motorsport Roller Rockers are banned but other mods allowed so it was fairly common to see a Kent based unit getting up there in the rev range and as others have said never ever seen one come loose. At one stage I had a Mk1 Escort big wing and we were running an Oselli Engineering 1300cc, that pulled 7,500 and 8,000 when I was dropping time, never had a valve issue.
