Flylow, nope, didnt use a lock washer, didnt see anything referring to a lock washer. I dont think using a lock washer would be a good idea.
For those of you that may not be familiar with what a locknut is. A locknut is a nut that has either been deformed to squeeze the threads or one that has a "plastic" insert that also squeezes tyhe threads of the adjoining fastner. We used these on full size aircraft (F-16s and F-4s in my experience) and I have never known a nut of this type coming off even when used on high vibration applications like the Vulcan Cannon, it spun fast enugh to spit out 6,000 rounds a minute, (100 a second) and never had one come loose.
I had a Harry Higley jam nut and a higley solid brass prop nut on mine when it let go the third time, it didnt help much. I had torqued it so tight I was afraid the threads might strip, it still spun off, thats when I decided to go the locknut way, never another problem.
A rule of thumb that we taught in full scale aircraft maintenance is a threaded fastner should engage, if possible, 1 1/2 times the diameter of the fastner. This is not always possible, but is the ideal formula for proper threaded faster engagment.
On the sucessful combination that has yet to fail to hold the prop on, on my OS 120, I used a 5/16-24 locknut, then since it was on a P-47, I wanted it to look somewhat like a full scale. I drilled partway (countersunk) into the back of the solid aluminum propnut so that all but approx 1/8 inch of the locknut was visible when they were both installed. This gave a somewhat scale appearance.
After all this I wondered why no manufacturer has made a solid aluminum or brass propnut that has the locking feature, that along with a higley jam nut would meet all the requirements and a somewhat scale appearance for a 4 stroke.
Denny