No, don't add split pins. That is unsafe. If the prop loosens up, the pins can damage the prop, and then it can disintegrate while running.

If I am understanding what you're doing, it may be the all thread. All thread is not very strong, and could easily be stretching and losing it's tension. You need to use at least a Grade 5 bolt. Tighten the prop, and then re-tighten it after sitting overnight. Another thing that could be happening is that your spinner adapter nut might be bottoming out and not allowing you to actually get the prop tight. If the spinner has damaged the prop, you need to replace the prop!

I don't like to use bolts for my props as you are wearing the aluminum threads each time you change the prop. I use long Grade 5 bolts to make my own "all thread". I clamp the head in a vise and tap the whole length of the bolt. Then I cut the bolt at the desired length. This gives a nice strong stud that I then install into the prop hub with Loctite. You can even drill the end of the stud for your spinner bolt if desired, but that should be done on a lathe.

(Note that to do this, you need to use a Grade 5 bolt that doesn't have a shank that is too large. If the shank is too large, you will need to turn it down to the proper size on a lathe before tapping the shank part.)

AV8TOR