I have also heard the same thing about the O.S. they have been replacing them even though there is no warranty on them. They will probably get the problem corrected if they are shelling out a lot of replacement motors..The Sirio is a little harder to tune. You can't go wrong with a RB motor..I have the Mbx-5 and run a Nova rossi P5 it ROCKS. I have seen them with the REX and a older model O.S. (RZ). they are close to the same.. The P5 has a little more bottom end than the O.S. if you run a tight track you might look into a REX or Rossi P5.