Drop-in resolution
Seiko 7S26 vs Seiko 4R35: do they swap?
Whether the Seiko 7S26 and Seiko 4R35 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko 7S26 (27.40 mm x 4.90 mm) and Seiko 4R35 (27.40 mm x 5.32 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 Seiko 7S26 is the long-running Seiko workhorse behind the SKX and Seiko 5, day-date, non-hacking. The Seiko 4R35 is the branded Seiko date automatic behind many Presage and Prospex models.
They differ in height (4.90 mm vs 5.32 mm), calendar layout (centre seconds, day and date at 3 vs centre seconds, date at 3), winding behaviour. The predecessor of the NH36 at 27.40 mm x 4.90 mm, without hand-winding or hacking. It shares the 27.40 mm case fit of the NH3x family, so an NH36 is the standard upgrade donor into a 7S26 case.
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.