Put the short arm up against its stop first and turn the shaft clockwise as far as it will go - then you need to be stretching the spring to hook the long end onto its ledge.
That's what I needed to do anyway, with my CL450 I didn't stretch anything, just put the spring where it seemed to fit straight away, and ended up with a limp shaft, oo er missus.
Trigger or Julie will tell you if this is b*****ks but that's what worked for me.