Hello eu comrade,
i have to ask, is 20,4mm the outside diameter?
I normally buy special sized orings on special shops for them, like HUG.
If its inner diameter is 20,4mm, you could try this:
Präzisions O-Ring 20,35 x 1,78 mm NBR90 ✓ Riesenauswahl im Online-Shop ✓ Großhandelspreise auch bei kleinen Mengen ✓ Versand sofort ab Lager! ✓ bis 65% Rabatt ✓
www.hug-technik.com
Maybe the chinese dont mind the 0,05mm and declare its a 20,4 but in reality its an 20,35....
If its the outside diameter, maybe this could possibly fit (i calculated 20,4-1,8-1,8 which results in 16,8mm inside diameter):
Präzisions O-Ring 17,17 x 1,78 mm NBR90 ✓ Riesenauswahl im Online-Shop ✓ Großhandelspreise auch bei kleinen Mengen ✓ Versand sofort ab Lager! ✓ bis 65% Rabatt ✓
www.hug-technik.com
Maybe it would be good if you measure the oring exact by yourself, so that you have clean results.