The Casio GA-2100, nicknamed the "CasiOak" for its octagonal bezel and integrated-bracelet styling, launched in 2019. In standard form it uses a Carbon Core Guard resin case, weighs roughly 51 grams, and carries a 200-metre water-resistance rating. A steel-cased mod keeps the watch's Casio module and replaces the resin case and bezel with machined stainless steel. The result looks and wears like a different watch. Here is what actually changes, and what does not.
What a steel mod is, and what it is not
A steel CasiOak is a genuine Casio GA-2100 with its outer case rebuilt. The movement, dial, hands and core functions remain Casio's. What changes is everything you touch: the case, the bezel, and usually the strap or bracelet. It is not a different watch internally, and it is not a Casio factory product. Any honest seller will tell you both of those things up front.
316L steel and the weight difference
Quality builds use 316L stainless steel, the same marine-grade alloy used across the watch industry for its corrosion resistance. The trade-off is weight. A resin GA-2100 sits at about 51 grams. A full steel case and bracelet can push the same watch beyond 120 grams. That heft is the point for most buyers, it gives the watch the presence its integrated-bezel styling promises, but it is worth knowing before you buy.
The honest part: water resistance
This is where most listings go quiet. A factory GA-2100 is rated to 200 metres because Casio seals and pressure-tests it as a complete unit. Once the case is opened and replaced, that rating no longer holds unless the finished watch is re-tested as a unit. We do not carry over the donor's 200-metre figure to a modified build. Treat a steel CasiOak as splash and rain resistant for everyday wear, and keep it away from swimming and diving. A re-cased watch still advertised at "200m" is quoting a figure that no longer applies.
Kit versus finished build
Most search results for a steel CasiOak point to parts kits sold on marketplaces. A kit is a case, bezel and tools; you supply the watch and the labour. A finished build is assembled, fitted and checked for you. The difference shows in the details: bezel alignment, gasket seating, clean screw threads, a strap or bracelet that sits flush at the lugs, and no marks on the module from the swap. A poorly fitted kit looks the part in photographs and disappoints on the wrist.
Why it is worth owning
The appeal of a steel CasiOak is honest: the durability and legibility of a modern Casio module in a case that reads as far more expensive than it is. Every ABA Custom build is assembled and inspected by hand at our West London bench, photographed as the piece you receive, and described with the same restraint we apply to everything we sell. We tell you it is a modified watch, what steel we use, and what its real water resistance is. That is the standard.
See the current builds in our Custom Series collection, including the Electric Blue steel CasiOak and the Heritage Cream build.