The Achilles heel for any mechanical watch is the winding crown. Setting the time, and winding the watch via the winding crown is a mechanical function held-over from pocket watches.  The winding crown is a separate moving part and before Rolex perfected it, the crown was a place for water and dirt to penetrate into the case rendering the movement unreliable. Hans Wilsdorf, Rolex’s founder, realized early that in order to keep a watch robust and sound, was to have a watch that wound itself to prevent wearers from having to pull out the crown, and a crown that was completely sealed.  Wilsdorf patented a sealable winding crown in 1926 and the automatic winding movement five years later in 1931.

The trick, Wilsdorf discovered, was to have the crown screw into the case and onto a rubber O-ring seal, similar to how a hatch closes in a submarine. When combined with a watch case that used a similar screwdown caseback, and screwdown bezel Wilsdorf had a watch case that was impervious to water, dust, and other gunk that killed watch movements.  Just to prove his point, Wilsdorf gave Mercedes Gleitze a Rolex to wear during her 10-hour swim across the English Channel, and his in-store displays featured a Rolex functioning inside a water filled fish bowl.
 

The Rolex Oyster case has evolved from a three-piece case into a solid piece of 904L steel, 18ct gold or platinum milled what we see in the Rolex line-up today, but the screw-in caseback and crown has not changed much since 1926. The screw-in crown exemplifying the submarine hatch concept is now industry standard that Rolex implements through two types of crowns, depending on their aesthetic and amount of water-impenetrability.  Rolex DateJust dress watches, women’s watches, and some professional models such as the Milgauss and Explorer I and Explorer II utilize the Twinlock winding crown that utilizes a double-system of O-rings that insures the crown of resisting water down to 100 meters / 300 feet.  An O-ring is fitted around the winding stem inside the case tube, and another O-ring fitted inside the crown that is pressed against the case tube when screwed onto the case.  The easiest way to identify a Twinlock crown is to look for a simple dash (-) or two dots (..) on the winding crown just below the Rolex five pointed logo crown.

Left to right, HEV, Chrono pushers like on Daytona, TwinLock, Triplock. Photo courtesy of Yazan Hatahet
 

Rolex’s professional watches such as diving, yachting, or multi-function watches, utilize the Triplock crown comprised of over 10 components made for this specific application and four O-ring gaskets that resists water at pressures at depths below 300 meters / 1000 feet.  With the Triplock crown, there is an O-ring seal outside of the case tube which the crown covers when screwed into the case, another inside the crown like the Twinlock crown, and two (2) O-ring gaskets around the stem tucked inside the case tube.  The Triplock crown is much larger in diameter, is identified by three dots (…) on the crown, but does not have the aesthetic for dress watches. When matched with a larger Oyster case found in the professional series, the Triplock winding crown looks right at home.  

 
A Rolex Oyster case is manufactured and fitted with a crystal, bezel, caseback and crown, it is subjected to water resistance testing to 10 times greater than the Rolex depth rating, and 30 times greater for diving watches.  This testing is a testament to Hans Wilsdorf’s pursuit for an accurate and reliable wristwatch in any environment and with care and maintenance, will outlast the life of its owner. 

-Sheldon Smith