Seiko NH38 (TMI NH38A) caliber: dimensions and drop-in group

Used in: the open-heart, no-date member of the NH3x family, in skeleton-dial mod watches.

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.

Drop-in group: Seiko NH38 open-heart, no date (27.40 mm x 5.32 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.