RVSuppliesPlus@RVSuppliesPlus.com
This is a fastener used in the assembly of the pump. A jam nut is a low-profile secondary nut tightened against a primary nut to lock it in place against vibration.
Always match by the MP Pumps part number 25168 listed on your equipment parts diagram, since dimensions and material vary across the MP Pumps lineup even for parts with the same common name.
Not sure this is the right part? Send us a picture of your Pump Plate to https://www.clrmarine.com/Contact-Us and the CLR Marine team will confirm the correct jam nut.