There should be a collar that goes on underneath the gear with 2 .5mm grub screws in it. If not........Pm me, I have a spare set I will send you. I will take a pic of the underside of mine and post it here.
To be edited!!.............................

Edit:
Lost........... I misdirected you.......... that collar comes with the MH strong steel shaft....... I have a extra set laying here I will send to you tomorrow. You do want the auto rotational gear, but of course you need a way to keep it in place. Pm me your vitals and I will toss it in the mail in the morning.........No charge as I am the one who made the mistake. It comes with 2 small spacers and one is wider than the other one......use the smaller one to get your gear mesh right. The thicker one is for the actual HDX300 heli frame. That space goes between your bottom main bearing and the gear. Pic 2 shows the set I am sending, let me know when you get it if ya need more help.


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Of all the things I ever lost....................................... I miss my mind the most!