Seiko 6R35 caliber: dimensions and drop-in group

Used in: the long-reserve Seiko date automatic, in higher-end Prospex and Presage models.

A 27.40 mm date automatic with a 70 h reserve, slightly slimmer at 5.25 mm than the 4R35. It sits in the same 27.40 mm case family but is a Seiko-only caliber, not sold unbranded as an NH.

Drop-in group: Seiko 6R date footprint (27.40 mm x 5.25 mm). 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.