The size of sonde housing to be used is determined by the size of motor you will be using. The sizes match up to each motor size. If you are using a 3″ motor, your sonde housing will be 3 to 3 1⁄2” OD. If you are using a 4 3⁄4” – 5″ motor, the housing will be 4 3⁄4” to 5″ OD. Most housings will accept either size of sonde housing: 15″ or 19″. The sonde housings are designed with a 4″ spacer to be added if you are using the 15″ and the 4″ spacer left out if you are using a 19″. Some Ditch Witch Sondes have larger OD’s then the normal and will not fit the space for the sonde.