Quick answer
A 42mm replica Speedmaster copies the Omega Speedmaster Professional Moonwatch, a manual-wind chronograph with a 42mm case. Case diameter is the easiest spec to reproduce, so it tells you almost nothing about quality. Grade a replica on movement architecture, subdial placement, bezel tachymeter print, crystal material, and caseback engraving. No current replica reproduces the genuine 3861 movement, its co-axial escapement, or its METAS Master Chronometer certification.
What the 42mm figure refers to
The Speedmaster Professional Moonwatch measures 42mm across the case, with the crown and pushers excluded, and runs about 13mm to 14mm thick depending on the reference. Omega also sells smaller Speedmasters, including the 38mm Speedmaster 38 and the discontinued Speedmaster Reduced at roughly 39mm. Sellers who advertise a "42mm Speedmaster" almost always mean the Moonwatch family: the 3570.50 and 311.30.42 references with the 1861 movement, or the current 310.30.42 references with the 3861.
Identify the exact reference number before you compare anything. A 42mm case with a date window at 3 o'clock is the wrong watch entirely, because the Moonwatch has no date.
Prerequisites
- A digital caliper that reads to 0.01mm
- A 10x jeweler's loupe
- Bright, diffuse light and a plain background
- Reference photos of the genuine reference number you intend to compare against
- The reference number of the watch in hand and the movement it is claimed to run
Inspection steps
- Measure the case at its widest point with the caliper, keeping the jaws clear of the crown and pushers.
- Measure thickness from the center of the caseback to the top of the crystal dome.
- Compare the distance from dial center to each subdial center against the genuine reference, since a movement swap shifts subdial placement.
- Read the tachymeter scale under the loupe and check the numeral font, the spacing, and the position of the 90 marker.
- Examine the dial print for the Omega symbol, the wordmarks, and the "Professional" line, checking stroke weight and letter spacing.
- Inspect the hour, minute, and chronograph hands for length, finish, and the counterweight shape on the chronograph seconds hand.
- Wind the crown and note the feel, then start, stop, and reset the chronograph several times to check pusher travel and hand return.
- Tap the crystal edge with a fingernail: hesalite gives a dull plastic note and sapphire gives a hard glass ring.
- Study the caseback engraving, whether a seahorse medallion or a sapphire display back, and compare text depth and finish to the reference photo.
- Weigh the watch on a kitchen scale and compare the figure against the published weight for the genuine reference.
Movement reality check
Genuine Moonwatches use a manual-wind column-wheel chronograph: the Lemania-based 1861 in older references, the 3861 in current ones. Most 42mm replicas run a Seagull ST19 or a related Chinese column-wheel clone, and some run quartz. The ST19 family is a real chronograph movement, which is why these replicas feel mechanical when wound, but it is not a Lemania and not a 3861.
Two consequences follow. Subdial spacing and hand length change because the movement sits differently under the dial. Service and parts support does not exist the way it does for a genuine Omega, so a failed chronograph module often means a full movement replacement.
Legal and practical notes
A replica is a counterfeit, and selling one in the United States violates trademark law. U.S. Customs and Border Protection can seize counterfeit watches at import, and the buyer loses both the goods and the money. Resale value sits close to zero, and an Omega service center will not touch the watch. Treat it as a novelty, not as an asset.