I've not had any problem with this shaft and gear by just letting the shaft protrude from the bottom of the auto rotation bearing about 7mm.

However, DON'T BUY THE HDX300 CNC HEAD from helidirect!!!!! The main hub does not fit the stock shaft or even their hdx300 shaft. It fits too loosely, so you can't get it on perfectly straight. This means you will always have a wicked wobble.

Ohyeah, when you mention to them that it doesn't fit the shaft, they stop replying to your messages [&o] I'm DONE with helidirect...