Drop-in resolution
Seiko NH34 (TMI NH34A) vs Seiko NH38 (TMI NH38A): do they swap?
Whether the Seiko NH34 (TMI NH34A) and Seiko NH38 (TMI NH38A) share a case, and what changes if you move between them.
Close in diameter, not a drop-in. The Seiko NH34 (TMI NH34A) (27.40 mm x 5.32 mm) and Seiko NH38 (TMI NH38A) (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 NH34 (TMI NH34A) is the affordable modding GMT automatic, adding a caller-GMT hand to the NH35 base. The Seiko NH38 (TMI NH38A) is the open-heart, no-date member of the NH3x family, in skeleton-dial mod watches.
They differ in calendar layout (centre seconds, GMT 24h hand, date at 3 vs centre seconds, open heart, no date). The GMT member of the NH3x family at 27.40 mm x 5.32 mm, adding a 24-hour hand to the NH35's date layout. It shares the NH35 case footprint and stem, so it drops into an NH35 case but needs a GMT dial and hand set. It is the unbranded Seiko 4R34.
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.