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.
Reassembling…
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.