Complete thread drift but there are bound to be incidents sometime if we use Imperial gallons, US gallons, litres, pounds and kilogrammes as refuelling measurements instead of an international standard!
Re the metric scale, almost every aircraft I've worked on over the last thirty years has used inches as a base unit, whether it's been British or American built. I'll admit that thread pitches and spanner sizes haven't been uniform though. I've never touched modern (to me, anyway
) widebodied aircraft, which may well be different re measurement units.
We do have wooden aircraft in Europe - the glue here in the UK was Aerodux (replaced casein glue, which deteriorated in damp conditions) if I remember rightly and I've worked with both Irish linen and Ceconite in regard of fabric coverings; it's no big deal if you take your time and know your dopes.
Oh, back to valve caps, according to my A&P manuals, they're used primarily as dust/dirt covers and secondarily as valve core leakage backups.