The gasket seal is where the physics meets the chemistry of aging, and whoever neglects this detail pays the price—late, but they pay. It's a ring or profile made of elastomeric material placed between two rigid surfaces—case and crystal, case and caseback, crown seat—designed to prevent water, moisture, and dust from reaching the movement. Without it, even the most robust case is an open box.
Materials have changed over time. Until the 1940s, lead or shellac was used; Omega introduced synthetic rubber gaskets in 1948, immediately raising the level of achievable sealing. Today's most common compounds are nitrile butadiene (NBR), also used in aerospace and nuclear applications for its resistance to hydrocarbons, and Viton® (fluoroelastomer), preferred where high-temperature tolerance and resistance to aggressive chemical agents are required. NBR and Viton are not interchangeable: each application has its own material, and installing the wrong gasket is worse than installing none at all.
Geometry matters as much as material. The L-profile gasket developed by Seiko for the Tuna—introduced in 1975—solves a specific problem in saturation diving: external pressure compresses the ring against both faces of the seat, reinforcing the seal instead of dislodging it. This geometry eliminated the need for the helium exhaust valve on those models. On the opposite end, solutions like the independent double gasket on the crown multiply the levels of protection; MB&F's HM11 reaches 19 total gaskets, of which 8 are on the crown alone.
The point I see ignored most often, both by collectors and certain repair shops, is degradation over time. A rubber gasket dries out, deforms, loses elasticity. A watch certified for 100 meters today can fall below that threshold within a few years if not serviced. The pressure test during overhaul is not optional: it's the only way to verify that the sealing claimed by the manufacturer still exists in reality. Installing a new gasket improperly—crooked, lubricated with the wrong compound, compressed beyond measure—produces the same result as not changing it at all. From the bench, you see it immediately: a gasket extruding out of its seat is a clear sign of sloppy assembly.
