Sirio, unlike all other engine manufacturers, makes their shafts with 1mm or more thread than normal. Got the evo 2 legal threaded shaft and had to use shims to make space for the flywheel, it went further 1 or 2 mm, but it was the only way to make it fit into my losi truck. then my buddy had a factory tc3 with a sirio econo 12, once he adjusted the flywheel, there was some thread visible that will not let the bearing rest against the flywheel. we had to use a rotary tool with a sanding disc to literally eat that extra 1-2mm thread and the problem was solved. this happened to the sirio 12 sg shaft. if you are using a centax clutch no problem. do not forget to use the collect included with the kit and not the engine!
by the way, I-m assuming that it is a sg shaft, if it is the standard but hand made cut short version , more sanding discs will be neccesary to make space to the flywheel.