posted by Nessism-FSA uses rubber coated crush washers to take up play in the system and account for differences in frame bottom bracket shell width. The crank bolt smashes down these crush washers when you tighten it. After time, the pinched rubber squeezes out and tension in the crank bolt decreases, which causes it to loosen - bolts stay tight because they stretch which keeps tension on the threads. A steel bolt with thread-lock should help but close monitoring of the bolt is still recommended in my opinion.
My understanding is that FSA has updated this design for their new top of the line cranks: the BB spindle is now tapered and the bolt has Loctite. Too bad they didn't update the entire line of cranks. FSA clearly doesn't do the testing and research that some other component company’s do. Buy their stuff at your own risk.-