Tom, I'm late on feedback but...you are talking sheer strength with the cleat right? It doesn't take much thread to hold for sheer. Pull strength is where you would be concerned with the thread. I think sheer on common 1/4" dia steel is around 1500-1800lbs. Pull tests I had done by an engineering firm for an insurance company on #12 steel sheet metal SCREWS in 20 gauge (I think it was 20) metal roof decking came out between 375-450 lbs per screw. They were 20 yrs old.