Tap water will destory a case or head over a short period of time, constantly refilling it wth tap water is the worse thing you can do, it will eat away the water passages, make it look like some weird worm creature ate thru the material.
Not to mention it leaves deposits and other yukkies along the way in radiator, this info also applies to ANY motor that runs coolant..car, truck, boat, fixed wing aircraft, dirigble, hoovercraft or Montgomery Wards Mini Bike (with water cooled head Mod of course)