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Fortis · Cosmonaut

Strap for Official Cosmonauts Chronograph Monolith

Takes 20 mm straps. Verified lug width, the straps that fit this model, and answers to the most common questions.

Lug width

20mm
20 mm strapsAll Fortis
Lug width20 mm
Case diameter42 mm
Reference638.18.31
FamilyCosmonaut
MovementETA caliber 7750 hours, minutes, small seconds | date, day | Chronograph
CaseStainless Steel

Spring bar

Swiss Standard universal spring bars

The right category for this model · 316L steel, all lug widths 13–26 mm

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Straps for the Cosmonaut 638.18.31

Selected for the 20 mm lug width · workshop selection

All 20 mm straps →

The Cosmonaut 638.18.31, seen from the bench

The Fortis Cosmonaut 638.18.31 is a functional chronograph with roots in Swiss aviation and space exploration instruments. The 42.00mm stainless steel case features a bidirectional bezel and highly legible dial with applied indices. It houses the ETA 7750 automatic movement with integrated chronograph (hours, minutes, small seconds, date, and day window). The 20mm lug width is calibrated for optimal proportions with this 42mm case diameter, requiring specifically sized straps for proper symmetry and contact with the wrist.

Frequently asked questions

What is the lug width of the Fortis Cosmonaut 638.18.31?

The lug width of the Fortis Cosmonaut 638.18.31 is 20mm. This measurement represents the distance between the strap lugs and ensures perfect compatibility with the 20mm artisanal straps available in the milanostraps.it collection.

Which strap is recommended for the Fortis Cosmonaut 638.18.31?

For the Cosmonaut 638.18.31, we recommend a vegetable-tanned alligator strap for formal wear, or a structured calf leather for versatile daily use. For a sportier aesthetic, a NATO strap or technical rubber strap harmonises perfectly with the chronograph's cosmonautic heritage.

How do I change the strap on the Fortis Cosmonaut 638.18.31?

The strap on the Cosmonaut 638.18.31 is changed using a spring bar tool: position the flat end of the tool under the spring bar from the inside of the lug, apply upward pressure to release the outer pin, and repeat on the opposite side. Align the new strap's holes with the lugs and reinsert the spring bars until fully seated.

Which aftermarket straps are compatible with the Fortis Cosmonaut 638.18.31?

All straps with a 20mm lug width are compatible with the Cosmonaut 638.18.31. milanostraps.it offers a complete collection of artisanal straps in leather, alligator, NATO, and rubber specifically crafted for this measurement, with various colour and finish options.

How long does a leather strap last on the Fortis Cosmonaut 638.18.31?

A high-quality artisanal leather strap, with regular maintenance (cleaning with a soft damp cloth and occasional protective balm application), typically lasts 3–5 years on a daily-worn chronograph. On a 638.18.31 worn intermittently, leather longevity extends considerably. Technical rubber, a more resilient alternative in humid environments, provides 4–7 years of intensive use.

The next step

The right strap isn't chosen. It is built, together.

From the atelier appointment to delivery, every step is made for those who know that things done well take time. Rare leathers, one piece at a time, eighteen months of warranty.

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Casati Milano · Via XX Settembre 15, Milan · by appointment only