18mm Nib Single The 18mm Nib has a spring valve system and an extremely durable felt tip. The measurement for this nib's diameter differs by system: 18mm (US measurement), 11mm (German measurement), or 10mm (Molotow). Please note: Although the 18mm Nib may appear similar to the Drip Mop Mini Nib, it is not compatible with the Art Primo Drip Mop Mini.