Drop-in resolution
Seiko 6R35 vs Miyota 8218: do they swap?
Whether the Seiko 6R35 and Miyota 8218 share a case, and what changes if you move between them.
Different footprints. The Seiko 6R35 is 27.40 mm x 5.25 mm and the Miyota 8218 is 26.00 mm x 5.67 mm, so they do not share a case. This is a choice between platforms, not a swap.
The Seiko 6R35 is the long-reserve Seiko date automatic, in higher-end Prospex and Presage models. The Miyota 8218 is a day-date 8200 member, in entry automatics that show the weekday.
They differ in diameter (27.40 mm vs 26.00 mm), height (5.25 mm vs 5.67 mm), calendar layout (centre seconds, date at 3 vs centre seconds, day and date at 3), winding behaviour. 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.
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.