Don't use cleaners with glycol ethers, either- that's about all of your simple green and wonder cleaners in a bottle- they also attack aluminum and yellow metals (though generally to a minor extent). I'd use regular dish soap. Also be aware that ZAMAK suffers from "zinc pest" or oxidation that can substantially reduce the integrity of the matrix. Ultrasonic may or may not yield unexpected results, depending on the overall condition of your ZAMAK parts. It's "probably" okay, but there's always a risk. You can't go wrong with a scrub with a stiff bristle brush and soap, but the usual tools for ultrasonic cleaning and the associated bottled products is iffy, double goes for if the cleaning bath is warm or hot.