Take a look at this thread:
My additional tip is if you have difficulty in threading the plastic end onto the bottom of the shock shaft:
I use an M3 tap which helps a lot and have read elsewhere of a 'bottoming tap' that allows a thread to be cut all the way down to the bottom.
But if you don't have such a thing, try a 6mm long M3 button head screw. Thread this into the plastic bit to help force a thread. You can actually tighten the button head screw all the way in and then even over tighten it a bit which opens up a thread and helps making threading the shaft on that bit easier.
This works for me and I hope it makes sense and helps - but if you crack your plastic shock end then YOU overtightened it!
