Another age-old dodge is to buy a much longer O ring, cut it, lay it into the groove, cut it again to fit then super-glue it together, this is exactly how professional O ring kits work for industry & I have done this countless times over the years with total success.
But for it to 'work' right, it has to be squarely cut & accurately lined-up when gluing.
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1965 Honda CP77 — Ulsan Metro City, Rep. Korea
Restored with original parts — Owner: J. Doe