Reviewing the Honda 250-300 shop manual from Bill Silvers CD I see mention of white and yellow cylinder head nuts.
I can see no difference in the cylinder head nuts on my CA77 engine, and the parts book only shows one part number with no white/yellow distinction.
Are there two different nuts?
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1965 Honda CP77 — Ulsan Metro City, Rep. Korea
Restored with original parts — Owner: J. Doe