Drop-in resolution
Seiko NH35 (TMI NH35A) vs Seiko NH38 (TMI NH38A): do they swap?
Whether the Seiko NH35 (TMI NH35A) 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 NH35 (TMI NH35A) (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 NH35 (TMI NH35A) is the default modding and microbrand automatic, hacking and hand-winding at low cost. 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, date at 3 vs centre seconds, open heart, no date). Larger than the Swiss core at 27.40 mm, so it will not drop into a case cut for a 25.60 mm ETA. It is the unbranded version of the Seiko 4R35 and shares that caliber's footprint and stem.
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.