Crystals put the receiver and radio on the exact same frequency. Radios and receivers are made for say a 72mhz band. you can then add say a channel 24 crystal and that makes them both on 72.27 mhz. Both parts have to have matching frequency crystal in them to work. In J.R. the receiver crystal is grey and transmitter is black. About 1/2 inch square and 3/4 inch long (just an estimate). They are required in all radios. If you buy a new 8103 radio they will have matching crystals in them.