bearings and ballscrew dont fit each other

[m_c]

As Gary says, far more likely the verniers are out. I've got a couple pair where the internal jaws are out (one set by about 0.5mm!), so if I'm using them, I zero them against a micrometer set to the required size.

One day I'll be able to afford a full set of internal micrometers...

[marbles]

Correct!! It must be that one side of the vernier jaws is out as I have to reverse them to measure the inside and out. Thanks for all the advice, much appreciated.

A bit of emery and on the bearings went. Worth asking the question thought as i'd not done this before and I didnt want to jam them on.

I wonder now if the retaining nut with the grub screw at the top of the twin bearing unit is enough to hold that lot, would a second nut be useful and if so where can I get one in the UK?