The safest answer is:
Replace the shaft so it extends the counter bearing.
But I will add this much based on my experience:
I have flown very hard 3D with no CB (counter bearing) on the X7 and many other heli's with 6mm shafts. Now of course I trim the shaft to not extend more than 3mm pass pinion. I have also flown some with shaft trimmed 3mm below pinion (sits 75% of pinion).
Here is what I feel the reason for the CB:
- improper gear mesh (too tight)
- shaft will flex on 5mm shafts
- 6mm shaft took care of this issue
- but due to higher HS and torque of bigger motors, CB would be ideal even with 6mm
If you properly mesh the pinion and main gear by giving just a hair of backlash on the high spot of the main gear, you are good to go. It's time intensive but well worth it.
So there are a few parts to your question:
Base on my previous experience, you can use your motor with shaft that extended only half way through your CB. Since it's OK w/o a CB and with this route, it's better and it's added safety. Just make sure you use some Green Loctite to keep shaft from spinning in the inner race of CB bearing. Make sure no Loctite gets in the bearing.
* Gear Mesh is critical