What is the Cartier Tank Must 31mm?
The Cartier Tank Must is a steel Tank with a quartz movement, a lacquered or guilloche dial, and a blue spinel cabochon crown. Cartier catalogues the steel line as Small at 29.5 x 22 mm and Large at 33.7 x 25.5 mm, so 31mm is marketplace shorthand rather than an official Cartier size name.
That phrase appears mostly on replica listing pages, which makes the size label itself the first clue to check.
How do you tell a fake Cartier Tank Must 31mm from a real one?
Authentication comes down to four things: case proportions, dial printing, the cabochon, and the movement. Fakes usually fail at least two of them.
- Case thickness. A genuine Tank Must is slim, about 6.6 mm on the steel Large model. Replicas built around automatic movements run 7.5 mm or more and sit high off the wrist.
- Second hand. The genuine watch is quartz and ticks once per second. A smooth sweeping hand means the replica uses an automatic movement.
- Dial print. Roman numerals should show sharp, even strokes. The word Cartier is worked into the VII numeral on many Tank Must dials; on fakes it is missing, malformed, or printed as a separate element.
- Cabochon. Genuine crowns carry a synthetic blue spinel, domed and deep blue. Replica stones look flat, milky, or plastic.
- Caseback. Engraving is deep and crisp with the correct reference and serial numbers. Shallow laser etching is a tell.
What does a fake usually get wrong?
Bracelet and clasp finishing, crown threading, and crystal quality are the common weak points. Sapphire with an anti-reflective coating is standard on the original, while many replicas use mineral glass that scratches and flares under light.
Weight betrays a replica as well. Genuine steel links and a solid folding clasp give the watch a density that alloy cases do not match.
What should you check before you buy?
- Ask for the exact case measurement in millimetres instead of trusting the listed size name.
- Request macro photos of the dial, cabochon, and caseback engraving.
- Confirm the movement type and reference number, then match both against Cartier's catalogue.
- Compare the asking price with the current market for that reference.
- Buy from an authorised dealer or a seller who supplies service records.
Is selling a fake Cartier Tank Must illegal in the US?
Yes. Trafficking in counterfeit goods is illegal under US trademark law, and buyers can face civil liability and seizure of the item. Counterfeit watches also carry no manufacturer warranty and often no serviceable parts.