The ETA 2824-2 is one of the most widely used automatic movements in modern watchmaking.
It is often treated as interchangeable with the Sellita SW200-1.
This assumption is incorrect at the level of case design.
Case geometry must be based on actual movement constraints.
Movement Overview
The ETA 2824-2 defines a fixed envelope.
Key Dimensions
- Diameter: 25.60 mm
- Height: 4.60 mm
- Stem height: 1.80 mm
These define:
- internal case diameter baseline
- crown tube centreline
- axial stack starting point
They do not define a complete case.
Interchangeability With SW200-1
The ETA 2824-2 and SW200-1 are dimensionally similar.
They are not identical in all contexts.
Assumptions of direct interchangeability can lead to:
- incorrect movement fit
- tolerance mismatch
- inconsistent retention
Case design must validate:
- actual movement dimensions
- supplier variation
- retention method compatibility
Movement to Case Fit
Radial Clearance
The movement requires controlled clearance.
Constraints:
- insertion clearance
- tolerance allowance
- retention interface
Too tight:
- assembly becomes inconsistent
Too loose:
- movement instability
- dial misalignment
Axial Stack
Movement height defines only part of the system.
Full stack includes:
- movement
- dial
- hands
- crystal
- caseback
Errors result in:
- hand interference
- sealing failure
- inability to close case
Stem and Crown Position
Stem height is fixed.
The case must align to it.
Defines:
- crown tube position
- case flank geometry
Misalignment leads to:
- stem binding
- increased wear
- keyless works damage
Movement Retention
The ETA 2824-2 requires controlled retention.
Methods include:
- clamps
- spacer rings
- integrated case features
Retention must:
- prevent rotation
- prevent axial movement
- avoid distortion
Incorrect retention leads to:
- positional drift
- functional inconsistency
Caseback and Sealing
Sealing is defined by axial stack and compression.
Constraints:
- gasket compression range
- thread or press interface
- tolerance stack
Errors result in:
- leakage
- inconsistent sealing
- assembly variation
Crystal Interface
Crystal position is part of the system.
Constraints:
- clearance to hands
- compression of gasket
- seat geometry
Failure results in:
- contact with hands
- sealing issues
- structural stress
Tolerance Considerations
Nominal dimensions are insufficient.
Design must account for:
- movement variation
- machining variation
- assembly variation
Tolerance stack defines:
- fit
- alignment
- sealing performance
What Goes Wrong
Common failures:
- incorrect assumption of interchangeability
- movement fit instability
- crown misalignment
- sealing inconsistency
- tolerance stack preventing assembly
These are not visible in nominal CAD.
Relation to Case CAD
The movement defines constraints.
Case CAD defines:
- how constraints are implemented
- how interfaces are controlled
- how geometry is manufactured
See: Watch Case CAD: From Movement to Manufacturable Geometry