Watch Case and Crystal Replacement

Case and crystal work looks like the easy part of watch repair, because nothing has to be timed. It is where most visible damage happens, because everything you touch is a finished surface and every tool you use is harder than it is.

Opening the case

There are two common back types and they need different tools. A screw-down back has notches around its rim and is turned with an adjustable pin wrench. A friction or snap back is prised at the seam with a thin blade. Using the wrong one on the other is how cases get scored.

• Identify the type before you touch it — look for notches around the rim
• Use a case holder; never hold the watch in your hand while forcing a back
• Work with the crystal facing down on a clean pad
• If a screw-down back will not move, check for thread lock, a dried gasket, or a bayonet fitting before using more force

The gasket is the whole job

Water resistance is not a property of the case. It is a property of the elastomer seals: at the back, at the crystal and inside the crown tube. Those seals age by taking a compression set — permanently flattened — and then hardening and cracking. A gasket that has been in a case for five years is not a gasket any more.

Replace gaskets every time the case is opened, without exception. Also inspect the seat: a scratched or nicked sealing face will leak no matter how good the new gasket is, and no amount of torque will fix that.

1. Remove the old gasket and inspect the seat under magnification.
2. Clean the seat; do not scrape it.
3. Fit the correct size and profile — inside diameter and section thickness both matter.
4. Grease lightly with a product rated for that elastomer. A gasket should look damp, not wet.
5. Check the gasket is seated evenly all the way round before closing.

Fitting a crystal

Sapphire is extremely hard — around 9 on the Mohs scale — and brittle. It resists scratching almost completely and fails by chipping or shattering under point load. Acrylic scratches easily but flexes. Mineral glass sits between them. The material determines how you can fit it.

Crystals are fitted with a press, using dies that apply even axial force. The upper die must be flat and slightly smaller than the crystal, so force goes to the crystal and nowhere else. The lower die must support the case by its band or lugs — never by the crystal.

• Press in short stages, checking alignment between each
• Use nylon or polymer dies on anything polished or coated
• Never press on a part of the crystal that is unsupported
• Replace the crystal gasket whenever a crystal is removed

Closing and testing

Closing is the reverse of opening, with one addition: screw the crown home. A screw-down crown left unscrewed gives no water resistance at all, whatever the rating engraved on the case says. It is the single most common cause of an unexpected leak.

Then pressure-test. A dry tester pressurises the case with air and watches for decay, or measures case deformation under vacuum; a wet tester looks for bubbles. Dry testing is safer, because a leaking watch does not get wet during the test.