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Sinn · Instrument Chronographs

Strap for Pilot Chronograph 903 St

Takes 22 mm straps. Verified lug width, the straps that fit this model, and answers to the most common questions.

Lug width

22mm
22 mm strapsAll Sinn
Lug width22 mm
Case diameter41 mm
Reference903.040
FamilyInstrument Chronographs
MovementSellita caliber SW500-1 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 Instrument Chronographs 903.040

Selected for the 22 mm lug width · workshop selection

All 22 mm straps →

The Instrument Chronographs 903.040, seen from the bench

The Sinn Pilot Chronograph 903.040 represents the modern evolution of Frankfurt's legendary aeronautical tradition. Powered by the Sellita SW500-1 automatic movement with column-wheel chronograph complication, this instrument watch combines Swiss certification with accessible service parts worldwide. The stainless steel case measures 41 mm in diameter with a 22 mm lug width, proportioned for reliable performance in cockpit environments. Unidirectional rotating bezel with precise 60-second increments ensures accurate flight time measurement.

Frequently asked questions

What is the lug width of the Sinn Instrument Chronographs 903.040?

The Sinn Instrument Chronographs 903.040 features a lug width of 22 millimetres. This measurement is taken between the inner shoulders of the case lugs and is precisely proportioned to the 41 mm case diameter of this chronograph.

What is the best aftermarket strap for the Sinn 903.040?

For the Sinn 903.040, we recommend straps in alligator leather with French or English cut, vegetable-tanned calf leather, or NATO in high-tenacity fabric. Alligator delivers structured elegance, vegetable-tanned calf develops natural patina over time, NATO ensures maximum functionality. Choice depends on use: formal-sportive for alligator, versatile daily wear for calf, extreme environments for NATO or rubber.

How do you change the strap on a Sinn Instrument Chronographs watch?

Use a spring bar tool to depress the small metal springs (spring bars) positioned within the case lugs. Press the tool inward toward the case interior to disengage the old strap's shoulder, then completely remove the spring bar and strap. Repeat for the opposite lug. Insert the new strap by aligning the holes with the spring bars, then release pressure until full engagement is confirmed.

Can I use a 20mm strap on a Sinn 903.040 with 22mm lugs?

No. A 20mm strap will not fit correctly into 22mm lugs. The 2mm differential will create visible gaps between the strap and the inner shoulder of the lug, compromising both aesthetics and structural stability. It is essential to respect the precise lug width: 22mm.

How long does an alligator leather strap last on a chronograph like the Sinn 903.040?

A quality alligator strap, with appropriate tanning and regular maintenance (gentle cleaning, conditioning every 12–18 months), maintains functionality and aesthetics for 5–7 years of daily wear. Longevity depends on climate (humidity accelerates deterioration), frequency of direct sun exposure, and regular water sports usage. Rubber and NATO straps offer superior resilience in extreme environments (3–5 years of intense wear).

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