Re: bk12 bearing eplacement
AC bearings can be purchased as a matched pair from China for very reasonable prices. A fussy youtuber ordered a set with low expectations but ended up surpised in a good way. If I come across that video again I'll link it here, he may have named the supplier.
Quality individual bearings off the shelf may be close enough to matched but in theory AC bearings should be bought in pairs. In theory as opposed to DIY reality..
Anyone who says "It only goes together one way" has no imagination.