Seiko watches have long been a favorite among horology enthusiasts due to their reliability, affordability, and modding potential. One of the most sought-after modifications is the GMT mod, which adds a second time zone function to your Seiko timepiece. This guide will cover everything you need to know about modifying a Seiko watch into a GMT, including the best base models, required parts, tools, and step-by-step instructions.

Why Modify a Seiko Watch to a GMT?

A GMT (Greenwich Mean Time) watch allows the wearer to track two different time zones simultaneously, making it an excellent choice for travelers and professionals who work across time zones. Seiko does offer some factory GMT models, but modding allows for personalization and the use of specific designs not available off-the-shelf. Additionally, modifying a Seiko watch provides a rewarding hands-on experience and enhances the value and uniqueness of the timepiece.

Choosing the Right Base Watch

The first step in a successful GMT mod is selecting the right base watch. Some of the most popular Seiko models for GMT modifications include:

• Seiko SKX007/SKX009 – A classic dive watch with robust construction and a versatile design.

• Seiko 5 Sports (SRPD series) – Affordable and widely available, with multiple style options.

• Seiko NH34-based watches – Already equipped with the NH34 GMT movement, requiring fewer modifications.

Required Parts for the GMT Mod

To modify a Seiko into a GMT watch, you will need the following components:

1. NH34 GMT Movement – This movement is a drop-in replacement for the NH35 and allows for a functioning GMT hand.

2. GMT Dial – A dial with a 24-hour track to properly align with the GMT function.

3. GMT Hand – An additional hand to track the second time zone.

4. GMT Bezel and Insert – Typically a 24-hour bezel to complement the functionality.

5. Replacement Hands and Chapter Ring (Optional) – If you want a completely custom aesthetic, you may consider new hands or a chapter ring.

Tools Needed for the Modification

• Case Back Opener – To access the movement.

• Hand Removal Tool and Hand Press – To safely remove and install new hands.

• Tweezers and Movement Holder – Essential for handling small parts.

• Crystal Press – If you are replacing the crystal.

• Spring Bar Tool – Useful for changing the bezel and strap.

Step-by-Step GMT Modification Process

1. Disassembling the Base Watch

Begin by opening the case back using a case back opener. Carefully remove the movement, dial, and hands using the appropriate tools. If your watch has an NH35 movement, it will need to be replaced with the NH34 GMT movement.

2. Installing the GMT Movement

The NH34 movement is a direct swap for the NH35, but ensure that all components align correctly. Secure the movement inside the case and verify its operation before proceeding.

3. Setting Up the GMT Dial and Hands

Install the GMT dial onto the movement, ensuring that it is properly aligned. Using a hand press tool, carefully install the hour, minute, seconds, and GMT hands. Ensure that the GMT hand is correctly positioned to follow the 24-hour format.

4. Installing the GMT Bezel and Insert

Replace the original bezel with a GMT-specific bezel that features a 24-hour scale. Some bezels are bidirectional, allowing for additional time zone tracking. Ensure that the bezel insert is properly aligned.

5. Reassembling the Watch

Once all components are installed, reassemble the watch by placing the movement back into the case. Secure the case back and check for proper water resistance if necessary.

Testing and Adjustments

After assembling the watch, it’s crucial to test its accuracy and functionality. Set the local and GMT time to ensure proper alignment and check for any hand misalignment. If needed, adjust the hands carefully using a hand-setting tool.

Customization Options

One of the best aspects of modding a Seiko GMT is the ability to customize it to your preferences. Some additional modifications to consider include:

• Custom Dials – Explore different color schemes and designs.

• Lume Upgrades – Enhance nighttime visibility with brighter luminescent coatings.

• Crystal Upgrades – Swap the stock crystal for a sapphire crystal with AR coating.

• Strap and Bracelet Changes – Experiment with different NATO, rubber, or metal bracelets for a unique look.

Final Thoughts

The Seiko GMT mod is a fantastic way to personalize your timepiece and add functionality not typically found in stock models. Whether you're an avid traveler or a watch enthusiast looking for a rewarding project, modifying a Seiko into a GMT is both a fun and practical endeavor. With the right tools and components, you can create a unique watch that suits your style and needs.

For more inspiration and watch modding guides, be sure to check out online communities and resources dedicated to Seiko modifications. Happy modding!