Uhmm. How do I say this.........

It dose fit in the lathe. I forgot the splined end was narrower than the 20mm shank at the taper. Just fits in far enough, about 25mm projecting from the chuck.

SORRY,SORRY, SORRY

Sounds like it has to reach up through the morse taper, past the taper knockout slot and then all the way up through the quill?
Not all the way through as this was the point, securing the collet chuck from the underneath, 90mm for the taper and knock out section + shank in chuck + depth of hole for new bolt thread. so long enough.