If you go the jam nut way, what you are after is a tight fit between the nut pairs, but be careful not to overtighten the outter nuts against the ball bearings.
Yup.
Found a bearing that has leaked a little with the old setup. Ordering replacements this weekend. I remember tightening that in frustration once a while back. 
Having problems getting the X-carriage to run smoothly along the rails with the new axles. Get binding as soon as I try tightening the lower wheels via the eccentric nuts.
Also one of the top wheels do not roll at all.
Reassemblingā¦
Also keep in mind that when using axles the adjustment of any eccentric spacer affects the whole axle and therefore both front and back wheels.
Adjust by little at a time, check both wheels, alternate sides, repeat. Stop when you feel resistance while spinning the wheels but can still do so with one thumb.