I did a little research and these are not a Japanese made bearing unfortunately. To extend life, do not spray with air or liquid in attempt to clean as this only pushes dirt into the bearing.
Sunday, Aug 30 2009 (about 10 years ago)
Mugen Seiki 15x21x4mm Bearing (2)
No problems with these bearings. I have yet to see them go out earlier than normal 8x16x5mm bearings. Not sure what everyone is talking about these bearings wearing very quickly, as I have yet to see it happen on any of the 4 Mugens I have built so far.