Casio G-Shock GA-B2100 CasiOak vs Seiko Presage SSA425 Open Heart
Casio's diver meets Seiko's timepiece
Quick Verdict
The Casio G-Shock GA-B2100 CasiOak ($100–$150) offers Casio Quartz (Solar-Powered) precision in the tier below the Seiko Presage SSA425 Open Heart ($250–$500). The G-Shock GA-B2100 CasiOak measures 45.4mm (48.5mm lug-to-lug, 11.9mm thick) and is rated 200m (20 ATM). The Presage SSA425 Open Heart counters with Made in Japan craftsmanship and 50m (5 ATM) water resistance. Both are exceptional watches for their respective tiers.
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Specifications Compared
| Feature | Casio G-Shock GA-B2100 CasiOak | Seiko Presage SSA425 Open Heart |
|---|---|---|
| Price | $100–$150 | $250–$500 |
| Case Size | 45.4mm (48.5mm lug-to-lug, 11.9mm thick) | 40.5mm |
| Movement | Quartz Module 5689 (Tough Solar + Bluetooth) | Automatic Caliber 4R38 |
| Caliber Type | Casio Quartz (Solar-Powered) | Seiko In-House Automatic |
| Power Reserve | About 7 months on a full charge without light; about 18 months in power-saving (Casio) | 41 Hours |
| Water Resistance | 200m (20 ATM) | 50m (5 ATM) |
| Crystal | Mineral Glass | Hardlex Crystal |
| Case Material | Carbon Core Guard Resin | Stainless Steel |
| Origin | Made in Japan | Made in Japan |
Category-by-Category Analysis
Design & Aesthetics
Casio's design language vs Seiko's approach
The Casio G-Shock GA-B2100 CasiOak features a Carbon Core Guard Resin case at 45.4mm (48.5mm lug-to-lug, 11.9mm thick) with Mineral Glass protection. The Seiko Presage SSA425 Open Heart offers Stainless Steel construction at 40.5mm with Hardlex Crystal. Both represent their brand's distinct design philosophy and heritage.
Movement & Performance
Quartz Module 5689 (Tough Solar + Bluetooth) vs Automatic Caliber 4R38
The G-Shock GA-B2100 CasiOak runs the Quartz Module 5689 (Tough Solar + Bluetooth) (Casio Quartz (Solar-Powered), About 7 months on a full charge without light; about 18 months in power-saving (Casio) power reserve) at standard frequency. The Presage SSA425 Open Heart uses the Automatic Caliber 4R38 (Seiko In-House Automatic, 41 Hours reserve) at 21,600 vph (3 Hz). The Presage SSA425 Open Heart offers a longer power reserve.
Wearability & Fit
45.4mm (48.5mm lug-to-lug, 11.9mm thick) vs 40.5mm — different wrist presence
At 45.4mm (48.5mm lug-to-lug, 11.9mm thick), the G-Shock GA-B2100 CasiOak has a bolder wrist presence compared to the Presage SSA425 Open Heart at 40.5mm. Water resistance differs: 200m (20 ATM) vs 50m (5 ATM).
Value & Cost of Ownership
Casio sits in the lower tier
The Casio G-Shock GA-B2100 CasiOak sits at $100–$150 and the Seiko Presage SSA425 Open Heart at $250–$500. On the specs that outlast any price: the G-Shock GA-B2100 CasiOak uses Mineral Glass with About 7 months on a full charge without light; about 18 months in power-saving (Casio), against Hardlex Crystal and 41 Hours on the Presage SSA425 Open Heart. Service intervals are typically 5-7 years for both.
Water Resistance
200m (20 ATM) vs 50m (5 ATM)
The G-Shock GA-B2100 CasiOak is rated to 200m (20 ATM) while the Presage SSA425 Open Heart handles 50m (5 ATM). The G-Shock GA-B2100 CasiOak is better suited for water activities.
Our Verdict
Casio
G-Shock GA-B2100 CasiOak
Choose the Casio G-Shock GA-B2100 CasiOak if you prioritize Casio Quartz (Solar-Powered) engineering, Made in Japan craftsmanship, and 200m (20 ATM) water resistance. At $100–$150, it delivers Quartz Module 5689 (Tough Solar + Bluetooth) with About 7 months on a full charge without light; about 18 months in power-saving (Casio) power reserve.
Check Price on AmazonSeiko
Presage SSA425 Open Heart
Choose the Seiko Presage SSA425 Open Heart if you value Seiko In-House Automatic technology, Made in Japan heritage, and 40.5mm proportions. At $250–$500, the Automatic Caliber 4R38 with 41 Hours power reserve makes it a compelling choice.
Check Price on AmazonThe Bottom Line
The Casio G-Shock GA-B2100 CasiOak and Seiko Presage SSA425 Open Heart represent two distinct approaches to fine watchmaking — the Casio bringing Made in Japan tradition while Seiko delivers Made in Japan engineering.
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How This Page Was Made
Our comparisons are generated from a hand-maintained specification database and reviewed under our editorial standards. We use AI tools to assemble pages from that data. We don’t invent specs: every number traces back to the database, and when we correct an entry, the fix is applied site-wide. As an Amazon Associate, The Horologist earns from qualifying purchases.