Nice frame! I always wanted one of those SLX Bianchis. what is the threading of the DA BB? are you sure it is Italian? I sort of agree with EX on this Bianchi used alot of English BBs on the Italian made bikes ( atleast the ones made for Bianchi USA)
there is no difference in taper between a DA, 600 or RX100 BB so any one of those with the correct threading will work.
the width od the shell is determined by the threading. it is possible that yours measures 69, and is undersized from manufacture or over zealous facing, or oversize from not being faced correctly. that is why BBs have 'adjustable' cups.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk