What pump can I use to replace a broken Myson Unit Five central heating pum?

I tried to run the heating boiler in my house and everything works fine except that the Myson Unit five pump was not working. I unscrewed the screw to release the flow and some water started to come out. When I tried to screw it back in, I couldn't. It remains turning in a band without pressing firmly so water continues to come out. Can you help me know how to tighten this screw properly?

Juan Campino
July 2020

grundfos 15-50's are the best ones

billybob
March 2010