A fake IW371640 usually fails on three points: case thickness, movement type and case-back engraving. The genuine IWC Portugieser Chronograph reference family measures about 41 mm across and roughly 13 mm thick, runs an in-house automatic chronograph calibre, and carries deep, crisp engraving on the back with a serial number that matches its warranty card. One clear mismatch is enough reason to walk away.

Prerequisites

  • A 10x jeweler's loupe
  • Digital calipers or a dial gauge
  • Bright, diffused lighting
  • Reference photos of a genuine IW371640 from IWC's own catalog
  • Access to a watchmaker or IWC service center for movement checks

Step-by-step authentication

  1. Read the reference number on the case back. Genuine pieces are stamped in full, in the IW371640 format, in a clean serif font. Replicas often show a partial number, a mismatched font or an off-center stamp.
  2. Measure the case with calipers. Confirm roughly 41 mm in diameter and about 13.1 mm thick. Many replicas sit 1 mm to 2 mm thicker because a generic movement is fitted inside.
  3. Weigh the watch head on a digital scale. A genuine steel case with sapphire front and back lands near 90 g to 100 g on a leather strap. A noticeably light piece points to plated brass or aluminum.
  4. Examine the dial under 10x magnification. Genuine applied indices have polished edges and sit flat. Counterfeit printing looks raised, fuzzy or slightly heavy, and subdial spacing at 12 and 6 is often off.
  5. Look through the display back. Expect an in-house IWC chronograph calibre with striped finishing, blued screws and an engraved rotor. Replicas usually show a rough, undecorated movement or a rotor with cheap printed text.
  6. Check the beat rate. The genuine chronograph runs at 28,800 vibrations per hour, which gives a smooth sweep. A quartz-driven fake ticks once per second.
  7. Operate the crown and pushers. Genuine pushers have short, firm travel with a clean click. The crown winds without grit and screws down flush to the case.
  8. Inspect the strap and clasp. Original IWC straps carry the reference number, size and a country-of-origin stamp on the lining. Deployant clasps should be signed and match the case finish.
  9. Cross-check the serial number against the paperwork. The number on the case back must match the warranty card and IWC service records. Ask for the original invoice and any service history.
  10. Have a watchmaker confirm the movement. A qualified technician can open the case and verify the calibre number in a few minutes.

Common tells on replica IW371640 watches

Watch for oversized or misplaced subdials, a date window on a reference that should not have one, a soft or incorrect Probus Scafusia crown logo, sapphire crystal without anti-reflective coating, and hands that are too short or too thick. Case proportions and lug shape are frequent giveaways, since replica cases are often copied from photos rather than measurements.

When to get professional help

Ask a watchmaker to inspect any piece offered well below market value, sold without papers, or from a seller who refuses an open-case inspection. If the serial number cannot be verified, treat the watch as counterfeit and stop the transaction.