The color codes are sub-sizes for the bearing. They usually range in size by only.0007". So even if your crankshaft.25 undersize you can precisely fit a bearing. If the crank is exactly.25 undersize, with no wear, then a green bearing is used. If the.25 journal has been worn a little, but still within spec., then one of the other colors is used. I'm assuming that green is exact size, blk is about.0002" thicker bearing , and red is the thickest bearing for a crank that is near the end of it's wear limit (.001" or.025mm).
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