Drop-in resolution
Sellita SW216-1 vs Sellita SW290-1: do they swap?
Whether the Sellita SW216-1 and Sellita SW290-1 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Sellita SW216-1 (25.60 mm x 4.35 mm) and Sellita SW290-1 (25.60 mm x 5.60 mm) are near in size but differ in height, dial feet or layout, so moving between them means a new dial, hands and often a case adjustment.
The Sellita SW216-1 is the hand-wound small-seconds caliber, in classic dress watches with a sub-seconds dial. The Sellita SW290-1 is the automatic small-seconds caliber with the sub-dial at 9, in dress and pilot watches.
They differ in height (4.35 mm vs 5.60 mm), calendar layout (small seconds at 6, date at 3 vs small seconds at 9, date at 3), winding behaviour. A hand-wound 11 1/2 ligne caliber with small seconds at 6 and date at 3, taller than the SW215-1 at 4.35 mm because of the separate seconds train. It needs a small-seconds dial and is not interchangeable with a centre-seconds movement.
Matching diameter and height is necessary but not sufficient. A donor is only a true drop-in when the dial-foot positions, hand-hole sizes, stem length and calendar layout also match. Two movements can share a case and still need a new dial, re-broached hands or a cut-to-length stem.