The operating nut is a large piece of metal with threads to attach to the upper stem.  The operating nut is the part of the hydrant that is "operated" or turned to open the main valve of the hydrant.  This piece of metal should be visible at the top of the hydrant.  Some operating nuts have a small hole with an oil screw on top.

If the operating nut is missing/broken, it will need to be replaced immediately.  As the hydrant cannot be used without it. 

We have an inventory of parts in stock and if you need us to repair your hydrant, contact us today. If we don't have the part you need, we can order it quickly for you and get your hydrant back in operating condition.