How to Assemble NH38

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How to Assemble a Watch with a Seiko NH38 Movement (Open Heart): A Step-by-Step Guide

The Seiko NH38 is one of the most rewarding and attractive movements for entering the world of hobbyist watchmaking or watch modding. Unlike its sibling, the NH35, the NH38 is an "Open Heart" (and no-date) mechanism. This means two things: it allows you to design watches where the balance wheel can be seen beating through the dial, and it is also much easier to assemble because it eliminates the complications of the date disc.

Here is the ultimate guide to assembling your watch with an NH38 from scratch.

🛠️ Essential Tools

For this project, you only need a basic assembly kit:

  • Movement holder: To securely hold the mechanism on the workbench.

  • Hand press or pushers: To precisely fit the hands without bending them.

  • Rodico: A sticky putty used to hold hands and remove fingerprints or dust specks.

  • Fine tweezers: With thin tips for manipulating small components.

  • Case back wrench: To hermetically seal the case upon completion.

  • Latex finger cots: Essential for avoiding grease or marks on the dial or mechanism.

🏗️ Step-by-Step Assembly Guide

Step 1: Removing the Provisional Stem

The movement comes from the factory with a gray plastic protective stem. One of the great advantages of the NH38 is that it does not have a "ghost position" (that intermediate position of the NH35 for changing the date). It only has two positions: in (running) and out (time setting).

  1. Place the NH38 in the movement holder.

  2. Locate the small metal release button (a point that only shows when the stem is in the normal running position).

  3. Gently press the button with fine tweezers and remove the plastic stem.

Step 2: Installing the Dial (Open Heart Dial)

Dials for the NH38 usually include a circular window (typically at 9 o'clock) to expose the textured balance wheel of the watch.

  1. Position the dial over the movement, aligning the window with the exposed balance wheel and the back pins with the holes in the gray spacer of the mechanism.

  2. Gently press down until it is completely flat.

💡 Revitalization trick: Dial feet are very fragile and break easily if the entry angle is not perfect. If the pins break, the dial is still usable. Completely remove the remaining broken pin and use small pieces of high-strength double-sided tape (or specific circular dial adhesives) on the plastic spacer to firmly and definitively secure the dial.

Step 3: Attaching the Hands (Much Faster!)

By not having a date complication, you skip the most tedious step of the NH35: you don't need to turn the stem to find the midnight jump. You can attach the hands directly.

  1. Hour hand: Position it pointing exactly to 12:00. Use the hand pusher to press it down evenly. It should be perfectly parallel to the dial.

  2. Minute hand: Also place it pointing to 12:00. Turn the provisional stem to complete a full rotation and verify that the hour and minute hands move in parallel, without rubbing against each other or touching the edges of the open heart window.

  3. Second hand: This is the most delicate due to its millimeter-sized arbor. Put a tiny bit of rodico on the tip of a toothpick to hold the hand from the back and position it over the central arbor. As soon as you feel it has engaged, press it firmly with the pusher.

Step 4: Fitting the Movement into the Case

  1. Remove the provisional stem by pressing the release button.

  2. Ensure the inside of the steel case is spotless.

  3. Insert the block (movement + dial + hands) through the back of the case, aligning the mechanism's channel with the case's crown tube.

Step 5: Cutting and Adjusting the Final Stem

The steel stem that comes with your crown is factory-made very long and needs to be adapted to the thickness of your case.

  1. Insert the long stem all the way and measure the exact excess between the case and the crown.

  2. Remove the stem and cut the excess with clean-cutting pliers (it's better to cut less and file the tip little by little).

  3. Apply a micro-drop of threadlocker (like blue Loctite) to the end and screw on the final crown. Insert it into the watch.

Step 6: Closing and Sealing the Back

  1. Place the black rubber gasket on the case thread (a touch of silicone grease will help maintain water resistance).

  2. Screw on the case back by hand, being careful not to cross-thread it.

  3. Secure the closure using the case back wrench, applying firm but controlled force.

Your custom watch with a Seiko NH38 heart is finished! Give the crown about 20 turns to wind the mainspring, set the time, and enjoy the hypnotic movement of the balance wheel beating directly on your wrist.