Honda motorcycles used to come with a small tool kit. The Hondas that had adjustable rear shocks came with a spanner to adjust those shocks. The Hondas are long gone but I still have a few of the spanners. If you can find one of those, it would probably fit your headset ring.
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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