This o-ring watch case back seal assortment is a great gasket kit for watchmakers and others who work on watch case backs. These round gaskets are made of solid tube rubber to create an airtight seal that prevents water from entering the watch case. They are flexible enough to fit not only round watch case backs but, square, rectangle and other fancy shaped case backs. As this assortment includes gaskets of various sizes, you should definitely be able to find a correct match for the gasket you need to replace.