Fortis
Cosmonaut 638.10.11— lug 20mm

The lug width of the Fortis Cosmonaut 638.10.11 is 20 millimetres. This measurement indicates the distance between the two lugs (case attachments) where the strap is secured. It is essential to verify this dimension before purchasing an aftermarket strap, as an incompatible width will not only appear aesthetically improper but will also compromise the functionality of the closure and place unnecessary stress on the case suspension pins.

For the Cosmonaut 638.10.11, we recommend alligator straps with vegetable tanning and medium-grain texture, or high-quality calf leather in shades such as burgundy, chocolate brown, or cognac. The squared, mechanical character of the case requires robust straps that complement the instrument-grade aesthetic of the watch. Alligator leather, in particular, ages naturally whilst developing a sophisticated patina that reinforces the watch's historical connection to aviation and space exploration. Avoid thin leathers or pastel shades, which would undermine the martial character of this model.

Strap replacement is a straightforward procedure: (1) position the watch with the strap facing you; (2) locate the two spring bar pins that run through the lugs; (3) use a spring bar tool to compress one of the pins and slide it outward; (4) remove the old strap; (5) align the new strap with the lugs; (6) reinsert the spring bar pin, compressing it slightly to verify tension. The operation takes less than one minute and requires no technical expertise. If you feel uncertain, most Swiss watch retailers offer this service complimentarily with strap purchase.

Absolutely. The Cosmonaut 638.10.11, owing to its stainless steel case and generous 20mm lugs, is ideally suited for high-quality aftermarket straps. Compatibility is guaranteed provided the lug width is precisely 20mm. Many owners prefer to maintain multiple straps—leather, NATO, and rubber—to diversify their wearing options. Milanostraps offers a curated selection of artisanal 20mm straps in various materials and colourways, fully customizable to your aesthetic preferences.

The lifespan of a leather strap depends on leather quality, frequency of wear, and environmental conditions. Straps in alligator or shell cordovan with vegetable tanning, worn regularly (4–5 times weekly) in normal conditions, typically last 4–7 years before requiring replacement due to natural wear. Vegetable-tanned leather ages favourably nonetheless, developing patina and increasing in suppleness with use. To maximise longevity, avoid prolonged exposure to salt water, intense perspiration, and direct UV light. NATO or FKM rubber straps, by contrast, maintain practical durability for 8–10 years with minimal aesthetic degradation.