The way I see it,when a person purchases an "upgrade/nonstock part " unless it comes from the heli mfg. that made the heli, one must be ready for some modification/adjustment to make it work.
If you wanted a "perfect "drop in you should have purchased stock parts,just my 2¢.
It's not that hard to make this work.Measure up from the top mast bearing on the stock shaft to the shear pin hole and transfer that to the new shaft to get the proper position,install the new upper retaining collor than install the shaft into the mast than gear and antirotation collor,mark the amount of shaft sticking out the bottom of the antirotation collor and trim. reassy. the mainshaft set your gear lash and your done.
parts setup is as follows---auto collor,gear---mast---steped collor with step to bearing,head
the lower tooth count will increase head speed some but not much