Drop-in resolution
ETA Valjoux 7750 vs Sellita SW510: do they swap?
Whether the ETA Valjoux 7750 and Sellita SW510 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The ETA Valjoux 7750 (30.00 mm x 7.90 mm) and Sellita SW510 (30.00 mm x 7.90 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 ETA Valjoux 7750 is the reference Swiss automatic chronograph, in a large share of mechanical chronographs. The Sellita SW510 is the tricompax Sellita chronograph, a 7750-family alternative with a 3-6-9 layout.
They differ in calendar layout (chronograph, subdials at 6-9-12, day and date at 3 vs chronograph, subdials at 3-6-9, date). A larger 13 1/4 ligne movement at 30.00 mm x 7.90 mm with the classic 6-9-12 subdial layout. Its drop-in donors are the Sellita SW500 and SW510, which copy its dimensions and subdial positions.
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.