We use what you need on our RC race boats to attach water cooling lines. #10-32 male thread on one end and a 1/8" barb on the other. You would still need to drill the end of the existing plastic part and tap it 10-32. Which means you would need a #21 drill bit, a #10-32 tap, and a tap handle.
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My C&V Bikes:
1972 Bottecchia Professional, 1972 Legnano Olympiade Record,
1982 Colnago Super, 1987 Bottecchia Team C-Record,
1988 Pinarello Montello, 1990 Masi Nuova Strada Super Record,
1995 Bianchi Campione d'Italia, 1995 DeBernardi Thron