worried the ridge on the plastic cup should go in flush to the bb shell
In an ideal world it would, but as long as it's providing proper radial support to the cartridge it doesn't matter that you seem to have run out of clean threads. The trouble usually arises because loose ball BB cups are typically thinner than cartridge BB cups, so the BB shell threads on older frames were only chased to the depth required by the older kit.
In an ideal world it would, but as long as it's providing proper radial support to the cartridge it doesn't matter that you seem to have run out of clean threads. The trouble usually arises because loose ball BB cups are typically thinner than cartridge BB cups, so the BB shell threads on older frames were only chased to the depth required by the older kit.