Ok, Let me correct my description and say it is the prop drive washer, not the thrust washer, that has the notch machined in it that is too big.
 http://www3.towerhobbies.com/cgi-bin...LXGCU9&P=Z
   There is space between the flat on the crank shaft and the flat on the drive washer and this lets the drive washer move. The noise it makes is a snap or sharp crack when using an electric starter or flipping the prop backwards against compression to start the engine.
 All 5 engine prop drive washers and one replacement part are all this exact same way.
 Fred