Airmar 04-004 51mm ID Hull Nut for (2-12in threads) Black Plastic 2inch (51mm) hole for 2inch outer diameter housings like Airmar P7 P8 P17 P19 P217 P314 P319 P617V DT800 DST800 DST900 ST300 ST550 ST600 ST650 ST850 ST900 ST950 235KhZ-Smart-Sensors 2.96875inch (75mm) outer diameter x 0.5inch (13mm) thick with 2inch (51mm) inner diameter hole