Tie Rod End Size

I have an adjustable track bar that uses the XJs Tie Rod End, but I'm missing the lock nut for it. Does anyone know what size the nut should be? I think the TRE is M18, but I'm not sure of the thread size.

Or do I really need the lock nut?

yeah you do. Take it to a tractor supply place or a fastener place. They should be able to help you out.