These ABEC 5 Blaze bearings from Sector 9 are made from high quality stainless steel and don't feature an inner shield making them super easy to clean. Fast, and clean, which is precisely how you want most things. Don't think about that last statement too much, it will crumble upon further inspection. However it is definitely how you want your bearings.
ABEC 5 - The ABEC scale is essentially a measurement of bearing tolerance for high precision bearings as dictated by the Annular Bearing Engineering Committee (ABEC). The scale runs from ABEC 1 through ABEC 9 using odd integers only. Essentially, the higher up on the scale a bearing is rated the tighter the tolerance is.
Tighter bearing tolerance means that the bearing moves with less resistance at high speeds, giving you a bit more roll for your push. In general most ABEC 3 or ABEC 5 bearings have steel races (shell of the bearing) with hardened steel ball bearings, and most ABEC 7 or ABEC 9 bearings feature steel races with super hard (much harder than steel) ceramic or titanium bearings.
Built-in Spacers - Built-in spacers allow you to tighten the axle all the way to ensure the bearings stay parallel in the wheel.