Although many bikes come without this part, you should always use one of these when repacking a cup-and-cone bottom bracket.
It protects the bearings and grease from crud that might be floating around inside the frame tubes, and will make your bottom bracket last longer.
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Bought this to replace the torn bb sleeve on my bike and its save me money having to replace rusted bearings. Buying a non servicable bb is great but im used to getting my hand dirty.