Here is the manual for your engine.
http://manuals.hobbico.com/osm/fs30s-40s-manual.pdf
They have a breakout view of the engine in the manual. It lists the OS part number as 45810100
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<font size="2">Doing a search on Tower Hobbies website comes up with the nut set you are looking for
http://www3.towerhobbies.com/cgi-bin...EARCH=45810100
The nut you are looking for is a standard thread of 1/4-28
If you want to get the Tru-turn adapter that will fit this engine, it is here
http://www.truturn.com/cgi-bin/store...mp;ppinc=adapt
Ken</font></font></p>