The prop nut you are using looks like it from an aluminum spinner setup. It could be that it is bottoming out on the crankshaft threads and therefor not putting any clamping force on the prop. You don't need that nut for your Dubro plastic spinner anyhow. I'd just use the original nut and washer that came with the engine. If you don't have that nut and washer, you can easily get replacements: http://www3.towerhobbies.com/cgi-bin...?&I=LXCP11&P=Z The link if for the .91 4-stroke nut and washer but it is the same 5/16-24 thread as the .65-LA and will work fine. (I didn't see a specific replacement nut for your particular engine)