My Saitos come with a lock nut. I get a large, flat washer, and 2 nuts. If you didn't get this with your 4 stroke, try to get another nut. Tighten the first nut real tight against the prop and flat washer. Hold this nut with a wrench, and install the 2nd nut. Run it tight against the first nut using another wrench. If your spinner uses a "center bolt" to hold the cone on, it can be used as the 2nd nut. I have 4 Saitos and have had some backfires while starting. I have never thrown the prop....... although one did loosen up ONE time. I also use Harry Higley's Lite Loc on a couple planes. It is a large, flat nut, about 3/16's" thick, made of aluminum. (he also makes a heavy model of brass). I use this right against the prop, and then use a prop hub nut run up tight against the flat Light Lock. I have never had a prop loosen on my .56 Saito all last summer with this set up. Of course...... check for a lean situation first like someone else mentioned......lownslo