The short answer
The 6654-1127-b97a leather strap is a tapered alligator-grain band built for a 40 mm complete-calendar case. Most examples measure 20 mm at the lugs and 18 mm at the buckle, with a padded profile and stitched edges. Fit comes down to two numbers, not the model name: lug width and buckle width. Measure both before you order, because a 1 mm mismatch shows at the case edge and puts strain on the spring bars.
Before you start
- Digital caliper or a ruler with millimeter marks
- Spring bar tool with a forked tip
- Soft cloth and a flat, clean work surface
- The watch case and the buckle you plan to reuse
Measure the lug width
- Set the watch face down on the cloth.
- Measure the gap between the inner faces of the two lugs at 12 o'clock.
- Repeat the measurement at 6 o'clock and confirm both readings match.
- Round to the nearest whole millimeter, since straps ship in 1 mm steps.
Measure the buckle width
- Measure the inner gap between the buckle prongs with the caliper.
- Compare that figure to the strap's narrow end, which should match within 0.5 mm.
- Note the reading so you can order a strap with the correct taper.
Pick the leather and lining
Alligator grain gives the dress look that suits this case, while a calfskin or goatskin lining adds lifespan against sweat. Check three details before buying: the stitching pitch, the edge finish, and the lining material. Machine stitching at roughly 3 mm pitch reads as factory work, and hand stitching at 2 mm reads as custom work. Painted edges resist moisture; raw edges age faster but look softer.
- Alligator or embossed alligator for formal wear
- Calfskin with a soft lining for daily wear
- Rubber or hybrid backing if you sweat heavily
Check thickness and spring bars
- Measure the strap thickness at the lug end and keep it under 4 mm so the band clears the case.
- Confirm the spring bars are 20 mm and rated for the case, not the strap.
- Compress each spring bar by hand to test the spring tension before installing.
Install the strap
- Fit one spring bar into the lug holes at 12 o'clock with the forked tool.
- Slide the strap end between the lugs and seat the bar in both holes.
- Tug the strap outward to confirm the bar is locked in place.
- Repeat the same sequence at 6 o'clock.
- Attach the buckle to the narrow end and check that the prong sits centered.
Break in and maintain
- Wear the strap for short sessions over the first week so the leather molds to your wrist.
- Wipe the strap with a dry cloth after each wear to remove oils and dust.
- Apply a thin layer of neutral leather conditioner every two to three months.
- Let the strap dry flat and away from direct heat if it gets wet.
When to replace it
Replace the strap when the lining cracks at the buckle holes, when the stitching frays, or when the leather stiffens and stops conforming to the wrist. A strap that has lost its padding will let the case sit off-center, which puts uneven load on the spring bars. Keep a spare on hand so the watch stays wearable while the old band is off.