That is the name of a "larger fitting" that hmmm refers to. It does the same job as a helicoil, but it's not a coil. It's a solid piece that has internal and external threads, much more solid and reliable than a helicoil. It looks like there's room for that here. And Crim is right, take it to a machine shop. If they say, "what's a keensert?" take it someplace else.