Why replace the case-back gasket
Most water infiltration in a watch comes from a damaged or badly seated case-back gasket. Every time the case is opened, this gasket must be checked and, if necessary, replaced. A dry, crushed or cracked gasket no longer guarantees water resistance.
Browse our watch case-back gaskets in various diameters and thicknesses.
Replacement steps
- Open the case and lay the case-back flat, inner face up.
- Remove the old gasket by pulling it with tweezers. If broken, lift every fragment from the groove.
- Measure the outer diameter of the inner lip of the seat with a digital caliper. Order a gasket about 0.1 mm below this measurement to guarantee correct compression.
- Measure the depth of the seat with the caliper depth gauge.
- Lubricate the new gasket with silicone grease (lubrication box or spatula application).
- Position the gasket on the case-back rim. Hold it with your thumb, stretch it slightly with your other hand and slide it into the groove.
- Check that it sits flat all the way round.
- Close the case applying even pressure.
Measurements and tolerance
For an O-ring:
- Outer diameter = inner diameter of the groove − 0.1 mm
- Thickness = groove depth + a small oversize for compression
Tools required
- Digital caliper
- Watchmaker's tweezers
- Silicone grease or lubrication box
- Screw-back wrench or sticky ball, depending on the case-back type
Everything is available in our watchmaker's tools section.
A case-back gasket costs less than a euro; ignoring it can destroy a movement worth several hundred.
Measurement reference table
| Measurement taken | Gasket to order | Tolerance |
|---|---|---|
| Case-back groove diameter | Gasket outer diameter − 0.1 mm | ± 0.05 mm |
| Groove depth | Gasket thickness + 0.1 mm | useful compression |
| Gasket inner diameter | Tube diameter − 0.2 mm | tight hold |
Common mistakes to avoid
- Choosing a gasket that is too large: it will pinch and lose its seal
- Forgetting to lubricate with silicone grease
- Closing the case without checking that the gasket sits flat
FAQ
How often should the case-back gasket be replaced?
Every time the case is opened, and at least every 2 to 3 years on a water-resistant watch in regular use.
Why lubricate the gasket?
Silicone grease preserves elasticity, improves the seal and makes fitting easier without pinching.
How do I know if the gasket is too small?
If the crown or the case-back feels loose after assembly, or if the watch picks up moisture, the chosen diameter is insufficient.
Will a water-resistant watch stay that way after this replacement?
The case-back gasket is only one element. Also check the crown gasket and the crystal gasket for complete sealing.