I have the thread-in hitch like you are describing, plus another one that I use instead (from Harbor Freight) which has the ball post actually welded into place. I check it every time I hook up and after 2 years, no cracks or evidence of impeding failure. Towing an M15 I don't worry too much about the stress, but I might be reluctant to use that type on a 17 or larger. -Neil