You are using way too much torque. First be sure the all the threads, BB and plastic cup, are perfectly clean then use grease on all threads. The fixed cup, (drive side) since its metal, needs to go on very tight, but the plastic cup doesn't need that much torque. It should be just snug with about about a 7 inch (3/8) ratchet.
Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1