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Seiko and TMI watch movements
Seiko, through its Time Module (TMI) arm, supplies the NH and 4R series automatics that dominate the enthusiast and modding world for their value.
Who Seiko and TMI are
Seiko makes movements both for its own watches and, through Seiko Instruments' Time Module business (TMI), for sale to other brands. The TMI NH35 and NH36 are the open-market versions of Seiko's own 4R35 and 4R36 calibers.
This split is why the same movement appears under two names. The NH35 sold to microbrands and the in-house 4R35 are essentially the same caliber, which is reflected in the paired pages here.
The 7S, 4R and 6R families
The 7S26 is the older automatic that neither hacks nor hand-winds. The NH35, NH36, 4R35 and 4R36 are its hacking, hand-winding successors and the default choice for modding and repairs. The 6R35 is a higher-grade caliber with a long power reserve and a reserve indicator on some models.
Each has its own page with the exact footprint, so a 7S26-based watch can be matched to an NH35 or 4R35 donor where the dimensions and fittings line up.
Seiko as the modder's donor
The NH35 is arguably the single most popular drop-in donor movement in the hobby, thanks to its low cost, wide availability and the features the older 7S26 lacked. The fitment tool shows which Seiko and TMI calibers share a footprint.
Because so many aftermarket dials and hands are made to Seiko fittings, a Seiko donor is often the path of least resistance, but the case bore and stem still have to match the caliber chosen.
Find the drop-in donor
Open the MovementSwap fitment toolMatch a caliber to the movements that share its case.Find seiko nh35 movement on eBayaffiliateRelated
Sources: Seiko and TMI (Wikipedia) ยท Movement datasheets (as cited on each caliber page)