I think its called the belly clamp? Not sure...its the metal ring that attaches to the outside to keep the lid on to the body. There is two washers, a spring, and a nut that you have to tighten to get the ring on nice and tight. Well, I took it all apart last summer when I cleaned out the filter and stored it for the winter. But for the life of me..my dh and I can NOT get that dang thing back together! We get the ring on, and get the bolt part thru the clamp, but can't get it squeezed enough to get the small hole washer-spring-lg hole washer and nut on there to tighten it up! What are we doing wrong? I need to fill the pool back up(its half full) so I can get bleach in it to circulate as there is green algae in it.