Use denatured alcohol or plain old paint thinner to degrease the bearings. Don't use one of the "magical, cleans all and everything" de-greaser/cleaners as many of them will etch the polished bearing surfaces or the bearing cage material.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now