Top wishbone bush - Ray Bull
 
  I saved myself alot of time and effort by using 3 x m18x1.5 steel nuts instead of trying to source the 18mm tap.

I screwed the three nuts onto the top ball joint thread so that when all nuts were held together the ball joint could be screwed in & out without bindind,at this point tack weld all the three nuts together and then remove the top ball joint.

Carefully fully weld the nuts together ensuring no spatter reaches the thread, you could protect this by fitting a M18x1.5 bolt . All you have to do is clean up the weld.

Total cost of nuts? £3.50 from a local hardware store.