futura,
I got real tired of replacing the main gear because the shaft was bent. I went to the Century main gear that has a replaceable shaft. So now if I do have a bent shaft I only need to drop it out the bottom without taking apart the whole heli. Now as far as the main shaft is concerned I ordered extras and I removed straight shafts from originals that the gears were stripped. So I have several now. The Century shafts are stronger than the original, I don't know why but I haven't bent one after the conversion.
Here are the sites where you will find the parts.
http://www.heli-world.com/Merchant2/..._Code=hum_cpfp
http://www.heli-world.com/Merchant2/...=parts-hb3dpro
If you use the main gear (and I recommend you do to give your motor pinion more to grip to) you will need to file down to 1mm the collar on the top of the main gear so it will fit tight to the bottom bearing. You will have to readjust the servo connections to readjust to the taller shaft. Here are some pictures showing the main gear and shafts.
Don