I just got off the phone with Castle's awesome tech support after questioning them about the water-resisting abilities of their Sidewinder ESC (which I just ordered). It turns out that, as I've read in other forums, the Sidewinder is "Highly Water Resistant." Basically what that means is that you can run it through water, but they don't say that for legal reasons. The guy I talked to said that they coat the circuit board in military-spec silicone sealant. That allows you to run it through freshwater just fine, and that it works perfectly for running through puddles, mud, and dirt with your truck. However, because saltwater is more conductive than freshwater, weird things happen when you run it through/under saltwater (i.e. it shorts out and breaks). Bottom line is, the Sidewinder is good to go in your truck if you want to run through the mud worry-free, and "go out and have some fun!" (Quoted from the tech support guy)
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