NH38 movement: technical guide
NH38 movement: open heart without a calendar, with hand winding and stop seconds. Compare TMI specifications and find MedoMods models fitted with NH38.
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The NH38A is TMI's three-hand open-heart variant: it has neither a date nor a day calendar. It is the movement to consider when the dial is meant to show a mechanical opening and a calendar display is not part of the plan.
The figures below are published manufacturer data, not an in-house test. In particular, the factory tolerance is not a promise of timekeeping for an individual finished watch.
NH38A technical data and calendar alternatives
| Specification | NH35A | NH36A | NH38A |
|---|---|---|---|
| Display | Three hands, date | Three hands, day and date | Three hands, open heart; no calendar |
| Automatic winding | yes | yes | yes |
| Frequency | 21,600 vibrations per hour (3 Hz) | 21,600 vibrations per hour (3 Hz) | 21,600 vibrations per hour (3 Hz) |
| Jewels | 24 | 24 | 24 |
| Power reserve | more than 41 hours with a fully wound mainspring | more than 41 hours with a fully wound mainspring | more than 41 hours with a fully wound mainspring |
| Factory accuracy | -20 to +40 seconds/day* | -20 to +40 seconds/day* | -20 to +40 seconds/day* |
| Movement diameter | 27.40 mm | 27.40 mm | 27.40 mm |
| Diameter with Dial Holding Spacer | 29.36 mm | 29.36 mm | 29.36 mm |
| Total height | 5.32 mm | 5.32 mm | 5.32 mm |
| Hand winding | yes | yes | yes |
| Stop seconds | yes | yes | yes |
*The 2022 NH38A specification states 23 ± 2 °C and a 20-second accuracy test for the same value. The later 2025 NH3 guide describes a static check 10–60 minutes after full winding in the dial-up, 9-o’clock-up and 6-o’clock-up positions, without its own temperature condition. These are not combined into one shared test condition and do not describe an individually measured watch on the wrist.
A dial opening is not a full skeleton
For the NH38A, “open heart” is TMI's name for the three-hand open-heart configuration. A dial opening frames part of the movement from the front. That alone does not make a dial a full skeleton display, whose design exposes the construction more broadly than one opening.
Choose the movement from the display you need. The NH38A suits the open presentation without calendar windows. If the dial needs a date, or a day as well, the NH35 vs. NH36 comparison puts those calendar choices in context.
Check components against the manufacturer drawing
The drawing keeps three reference dimensions separate: 27.40 mm for the movement, 29.36 mm with the Dial Holding Spacer and 5.32 mm total height. Check the dial-opening position and dial feet, hand fittings, movement holder, case, winding stem and clearance beneath the crystal against the drawing and component data. The spacer diameter is not a substitute for the bare movement diameter.
Two NH38 open-heart models
These MedoMods models use an NH38 movement with an open heart:
For a build of your own, the configurator shows the available options.
Source basis
The specifications come from the TMI Cal. NH3 Technical Guide & Parts Catalogue, Version 03 of 25 June 2025, the TMI catalogue, and the NH38A specification and drawing, revised 16 November 2022. The sources were retrieved on 09 October 2026.
FAQ
Does the NH38 movement have a calendar?
No. TMI lists the NH38A as a three-hand open-heart movement without a date or day calendar.
Which NH38A dimensions matter when choosing components?
The TMI drawing lists a 27.40 mm movement diameter, 29.36 mm with Dial Holding Spacer and a 5.32 mm total height.
Is the NH38A factory accuracy a promise for every watch?
No. TMI states -20 to +40 seconds per day under static test conditions; it is not a measurement or promise for an individual finished watch.
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