JoeHanson said..

When building wheels, how much undersize for the hub is appropriat to fit the ballbearings in?
My father suggests 0,5 mm, but i think less would be better.
Joe

I assume you are making a nylon plug to fit in a mold? or a machined ally hub in the middle of your wheels?
when i make the nylon plug for the mold the difference is 0.1mm. this. If you make the hug in glass/polyester the shrinkage will be just right.
if its carbon/polyester its a tiny bit(tad) tighter, if its carbon/epoxy its 2 tads tighter. . with the carbon hubs I use a sharp stanley knife and just scrape a thin shaving off the finished hub. it should come out as powder, not a shaving. this imprves the grip on the bearing also.
if the hub gets loose with use I simply clean out with acetone,rough up and wipe on some fresh epoxy next time I have some mixed

Joe They should not be super tight just a tap in fit is fine Tapping a piece of tube that is slightly smaller than the outer and making sure the bearing is square usually does the job If you cant find a suitable piece of tube a socket of the right size from a socket set can work
Do not tap on the inner part as this is bad for the bearing and can chip the balls inside
It does sound as though the machined areas are slightly out of line You can probably correct that by making the housings a loose fit and putting the bearings in with Loctite or a similar product when the axle is in place They have a product called Bearing mount but there are other products as well
Good luck with it
cheers Hiko