Well, despite using Loctite, nothing has improved because the crescent-shaped piece in the crankshaft stub is slightly worn (it's really only a slight wear, so that when the engine is warm, the material expansion might be enough to make it fit properly). Have you ever tried compensating for the tolerance using flowable metal?
Oh yeah... or does anything else come to mind ? I just don't want to completely replace the crankshaft, otherwise I might as well overhaul the whole engine, and that's really not affordable for a student (not even the material costs...) .