BB as 2slow suggested to fly with only half of the prop nut theads engaged is a real bad idea. On the groung the first time you hit the starter slightly flooded the prop and spinner will be flying through the pits. The first time in flight that you get a lean run likewise the prop and spinner will depart the airplane.

Much more functional and actually more attractive are machined spinner nuts. There is no back, plate the spinner nut has a machined washer face so a washer is not needed. The cone shaped one fits most starter inserts far better and you will never have to put up with that ugly black ring from the starter that always occurs with plastic spinners. Also plastic spinners if not properly fitted will break up when a starter is applied, this is a frequent occurance.

This what they look like and your Fx .61 Uses the 5/16 24 thread size.

http://www2.towerhobbies.com/cgi-bin...&I=LXE158&P=ML