After a considerable amount of swearing, I've FINALLY got my new spindle in my SPG0618A headstock. When dinner goes down, I'm going to see about mounting on the bed.
The original spindle was extracted to replace the plastic gears which broke on me. It was ridiculously stiff and I ended up persuading it with an 'engineers friend' Later I became convinced that this was a bad idea. The taper appears to have been damaged and it's impossible to get a good finish on steel.
Getting the spindle out was a challenge. I'd made a press out of some scrapyard I beam – which works but turned out to be too small, so I had to take it apart and stretch it.

It still needs some fettling as it rattles around somewhat – I need a collar or something round the screw.
I've replaced the bearings with angular contact bearings and was concerned about how tight the originals were on the spindle. I asked in another thread about 'push fit', the concern being that the bearings were jumping in 0.5 – 1mm increments when pressed on or off, so hard to get sensible pre-load.
ARC's pictorial says to polish up the old spindle on your lathe. Ah. Apparently you need a lathe to fix a lathe!
Using a bit of lateral thinking I mounted my little 4 Mt2 4 jaw in my little mill and found I was able to polish it up nicely.

The fit is still tight (i had about 6 trials – not wanting to take too much off), but it pushes on smoothly without any creaking or jerking.
When I came to refit, I found that the spindle gears wouldn't push on, so out came the gear idler and a bit more polishing on the key. I got the spindle gears mounted in nicely and then realised I should have put the gear idler back in first whilst I could get to it.
After all this I really didn't fancy getting the spindle out again, so a bit more drilling and jig making and I've finally got it all together.
Now to hope that it's reasonably in line when I get it back on the bed and I shall be happy. If you don't hear from me by the end of the weekend, it's probably all gone horribly wrong again…
Iain