Ducati 125 Sport 160 Monza Exhaust Nut

Ducati's part number for this nut is 0160-84-030, I believe.  It should fit just about all small Ducati OHC single models to 160cc.  ID is 36mm.

This beautiful nut is cast in BRASS, just like the originals, then chrome plated.  Unplated versions also available, for the vintage look you may want.