Drop-in resolution
Seiko NH38 (TMI NH38A) vs Seiko 6R15: do they swap?
Whether the Seiko NH38 (TMI NH38A) and Seiko 6R15 share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko NH38 (TMI NH38A) (27.40 mm x 5.32 mm) and Seiko 6R15 (27.40 mm x 5.25 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 NH38 (TMI NH38A) is the open-heart, no-date member of the NH3x family, in skeleton-dial mod watches. The Seiko 6R15 is the predecessor of the 6R35, a date automatic in earlier Prospex and Presage models.
They differ in height (5.32 mm vs 5.25 mm), calendar layout (centre seconds, open heart, no date vs centre seconds, date at 3). The open-heart, no-date member of the NH3x family at 27.40 mm x 5.32 mm, sharing the NH35 footprint and stem. It drops into an NH35 case but needs a dial with an open-heart aperture and no date window.
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.