Answer First
A fake IW371651 is a copy of the IWC Portugieser Chronograph reference IW371651, and you can judge one in about ten minutes with three checks: measure the case with a caliper, read the dial print under 10x magnification, and inspect the chronograph movement through the sapphire caseback. Genuine IW3716-family watches use an in-house IWC chronograph calibre with a column wheel, and most copies miss on case thickness, subdial finish, and the snap-back of the chronograph reset. Confirm the exact published spec for IW371651 on IWC's own reference page before you compare, because the Portugieser Chronograph line has changed dial and case details across production runs.
What you need
- Digital caliper that reads to 0.01 mm
- 10x loupe or a macro lens
- Bright, diffused light and a clean microfiber cloth
- Photos of the reference from IWC's site or a dealer listing
- A watchmaker who will open the case if you need a final call
Step-by-Step Check
- Set the watch flat on the bench and measure case diameter across the bezel at 12 to 6, then at 3 to 9. Both readings should match the published figure for IW371651.
- Measure case thickness from the caseback center to the crystal top with the chronograph running. Copies usually run thick here, often by half a millimeter or more.
- Weigh the head without the strap and compare it to a known genuine figure for the reference.
- Put the loupe on the dial text and check that each letter edge is crisp, evenly spaced, and free of ink bleed or double-strike blur.
- Inspect the applied numerals and hour markers for flat, polished tops, sharp inner corners, and no gap where the marker meets the dial.
- Look at the subdial at 6 and confirm it is a running small seconds, then check the subdial at 12 and confirm it counts to 30 minutes.
- Examine the subdial rings for a fine, concentric snailing pattern that catches light evenly, not a flat printed disc.
- Check the hand finish: leaf-shaped hands should show a clean center ridge and polished bevels, with tips that stop short of the minute track by the same margin on every hand.
- Turn the crown and confirm it is signed and that winding is smooth with no grit.
- Press the start pusher and note the effort and the sound. Genuine column-wheel chronographs feel light and click once at start and once at stop.
- Run the chronograph for thirty seconds, stop it, then press reset. The hands must fly back to zero together and stay there. Creep or a soft reset points to a swapped chronograph module.
- Flip the watch and inspect the caseback crystal for a blue or purple reflection from anti-reflective coating on the outside.
- Read the movement through the caseback and look for Geneva stripes with even spacing, a signed rotor, and a column wheel visible near the pusher side.
- Check the caseback engraving depth and letter spacing against your reference photo, including the reference number and serial.
- Inspect the strap stitching and the buckle stamping, then confirm the buckle signature matches the case finish.
- Ask for the original papers and have the serial confirmed by an authorized IWC service point before money changes hands.
Red Flags That Decide It Fast
- Case thickness that runs over the published figure by 0.4 mm or more
- Dial print that softens or bleeds under 10x magnification
- Subdial rings that are printed flat instead of snailed
- Chronograph reset that drifts off zero or feels spongy
- Movement that shows a decorated plate over a third-party chronograph base
- A price far under the going rate for the reference, with no service history
After the Check
- Photograph the caseback, dial, and movement through the loupe so you have a record of what you saw.
- Take the watch to a watchmaker for a movement identification if any check is borderline.
- Walk away from any deal where the seller blocks an inspection or a serial check.
One clean inspection beats any claim in a listing. Measure, magnify, and run the chronograph, and the reference will tell you what it is.