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@moth
So long as i could successfully install the cup i'd just not worry about a lone crack, or about more cracks that develop in-situ. The worst thing that could happen is it might disintegrate when next uninstalled, but there's no reason to hasten that day by replacing it early, and it's only plastic so not going to damage anything else if it does break up.
(while UN54 bb's come with plastic cups, UN55's have aluminium ones. Waving my calipers at them it seems they work slightly differently. The plastic ones have a shallow internal taper that begins to engage the sealed bearing when half-way installed and the far end of the taper is about half a mm smaller than the bearing, so it gets increasingly difficult to turn a you do it up and it probably doesn't matter if you don't get it all the way on as long as it's tight.. In contrast the aluminium ones have a sharp step that bears only on the very end of the bearing, so turn easily until fully done up.)