In order to run your radio control car , plane or boat you need to make sure that you are the only one on the Radio Control Frequency or someone else might control your car , plane or boat resulting in loss or damage . so it advisable to have more than one set of crystals so you can change to a clear frequency if there is likely to be a clash . FIND YOUR CRYSTALS HERE If yyou buy more than one set , ignore initial invoice as it may show too much post charge