Quick answer

A fake Omega Seamaster 300M is judged on five checkpoints: case profile and crown guards, bezel action and insert, dial printing and date window, movement behavior, and bracelet end link fit. A budget copy misses at least three of them. A high-grade replica gets the case silhouette and dial finish right and runs a serviceable automatic movement. The 300m stamp on any replica dial is decoration, not a dive rating.

Check the case profile first

The current Seamaster Diver 300M uses a 42mm case with lyre-shaped lugs and a helium escape valve at the 10 o'clock position. Cheap copies often substitute a generic 41mm or 43mm case that sits flat on the wrist and wears wider than the numbers suggest.

  1. Measure the case across the 3 and 9 positions with calipers. A 1:1 replica should read 42mm.
  2. Look at the lug curve from the side. Genuine lyre lugs sweep down and inward. Low-grade cases cut straight down from the bezel.
  3. Inspect the helium escape valve at 10 o'clock. It should sit flush with the case flank and carry a crown of its own, not a glued cap.
  4. Run a fingernail along the crown guards. Sharp, unfinished edges point to a low-grade case.

Inspect the bezel

Current models use a ceramic insert with a white enamel dive scale and a lume pip at 12 under a raised ring. The bezel turns one way only, with 120 clicks and no back play.

  1. Turn the bezel a full rotation. Any backward movement between clicks means a worn or cheap click spring.
  2. Check the scale under a loupe. Enamel fill should sit level and stop clean at the edges.
  3. Press the lume pip with a soft tool. Painted dots flex or flake.

Read the dial and date window

Dials from 2018 onward carry laser-cut waves. Earlier references are smooth, so match the dial style to the reference you are comparing against.

  • Printing: "Seamaster" and "Professional" should be crisp with no double edges or blur under 10x magnification.
  • Date window: the aperture sits at 6 o'clock and the cyclops, when present, centers over the date without a halo.
  • Hands: skeletonized hour and minute hands, lume filled and even, with no paint spill on the polished edges.
  • Logo: the applied Ω symbol should sit level and reflect without a visible seam.

Test the movement

A genuine current example runs a Co-Axial Master Chronometer Calibre 8800 at 25,200 vph with a 55-hour power reserve. Clones rarely match that, so test what you can.

  1. Wind the crown 30 turns. Grit or grinding signals a cheap gear train.
  2. Pull the crown through positions 2 and 3. Hacking should stop the seconds hand clean.
  3. Set the watch on a timegrapher. A 28,800 vph beat rate is normal on a good clone. A 21,600 vph reading is a budget unit.
  4. Let it run overnight off the wrist. Anything under 30 hours of reserve is weak.

What the 300m number actually means

ISO 22810 sets the test requirements behind a watch depth rating, and those tests cover gaskets, caseback threads, and crown seals as a system. A replica may carry a 300m stamp, but the seals are not tested to that standard and the caseback threads are often cut loose. Treat any replica as splash resistant. Rinse it after contact with salt water, do not press the crown under water, and never dive with it.

Check the bracelet and clasp

  1. Look at the end links against the lugs. A gap wider than a hair shows a mismatched case and bracelet.
  2. Push the clasp closed. It should click once and lock with no rattle when you shake the watch.
  3. Check the screws on the links. Stripped or soft heads mean cheap steel.
  4. Flex the bracelet. Stiff links with rough edges will wear the pin holes fast.

Final checklist before you buy

  • Case measures 42mm and the lugs curve inward.
  • Bezel clicks 120 times in one direction with no back play.
  • Dial print is sharp and the date sits centered at 6 o'clock.
  • Movement hacks, winds smooth, and holds over 30 hours.
  • End links sit tight with no visible gap.
  • Seller states the movement type and offers service parts.