Strap for Official Cosmonauts Chronograph Mars 500

The lug width of the Fortis Cosmonaut 638.28.13 is 20 millimetres. This measurement corresponds to the internal spacing between the two lugs of the case, where the strap is secured via the spring bars. To verify this, simply remove the strap and measure the distance between the lug seats using a digital calliper.

There is no single 'best' strap, as selection depends on intended use. For formal and semi-formal settings, an alligator leather with vegetable tanning enhances the watch's character. For tactical daily wear, a nylon NATO in grey or navy blue preserves the military identity. For humid or sports environments, an FKM (Viton) rubber strap ensures durability and low maintenance. A full-grain calf leather represents the versatile compromise, developing natural patina over time.

Strap replacement is executed by extracting the two spring bars that secure the strap to the lugs. Use a dedicated spring bar tool to slightly lift the lever of the bar, then slide the strap outward. Repeat on the opposite side. Insert the new strap by aligning the terminal holes with the lug seats, then reinstall both spring bars until the locking lever clicks into position.

Yes, the 20mm strap is compatible with any watch sharing the same lug width. However, strap thickness must match the case proportions: excessively thick straps may not seat correctly on slender cases, and vice versa. MilanoStraps supplies 20mm straps calibrated for various case geometries, with technical guidance for proper selection.

Lifespan depends on frequency of use, climate conditions and maintenance. An alligator or vegetable-tanned calf leather, kept dry and regularly cleaned with a damp cloth, typically delivers 3–5 years of daily wear before replacement is needed. Visual wear begins after 18–24 months, with edge fraying and slight colour loss. The natural patina that develops does not indicate functional degradation, but rather aesthetic ageing that many enthusiasts regard as desirable.