A screw-down caseback is a back cover that screws into the watch case. The threads hold it firmly in place and give a secure seal. This improves water resistance, so screw-down backs are often fitted to dive watches.
How does it compare with a snap-on caseback?
A snap-on caseback comes off more easily than a screw-down one. That makes it simpler to open, but it may keep water out less well. A screw-down back is fixed by its threads, so it takes more effort to remove. The screw-down back is one of several ways to attach a caseback. Others include bayonet mounting and fixing the back to the case with several screws.
Where it comes from
The screw back was not a Rolex invention. One specialist writer says screw backs and bezels were already well-established technology when the Oyster appeared. Borgel made screw cases before the Oyster. On those earlier Borgel cases, very fine threads made a tight seal that kept out dust and moisture. Rolex introduced its first Oyster case in 1926 and promoted it as a new hermetically sealed design. The same writer considers the Oyster case almost certainly inspired by Borgel’s three-piece screw case of 1903.
What made the Rolex Oyster case different?
In the Oyster, a threaded construction pressed the caseback, case middle, crown and crystal tightly together. The screw-down back was one part of that closed system. The case shut like a shell, and the Oyster name comes from that closure. Rolex later made its casebacks thicker. It also added patented expanding ribs on the back, so that the backs could withstand high water pressure.
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