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Quoted from "howie"

created a lot more work for myself!

Quoted from "howie"

I know it's an old thread, but I just dealt with the exact same issue, and wish I had some of this information in advance. Those brass Nelson heads are attached to a 1/2" male thread adapter onto which you can screw a plastic head. The bottom of that adapter has the odd female thread size which you mention, and which can't be found at stores. That female thread screws onto a brass threaded riser, soldered into a copper T. You need to remove the adapter from the old sprinkler head, put a new head on top, and thread it back onto that riser.
So that adapter is like GOLD when you are trying to repair these! If you can reuse it, it's pretty easy to pop a new head on.

The metal threads on that adapter will however shred plastic sprinkler bodies easily. So you might want to take a metal pipe fitting and use it to clean up the adapter threads, so you cane hopefullt get a plastic body on without cross-threading. Or, first put a new piece of metal on that old brass adapter, then the plastic head. If the adapter is underground, and you try to thread plastic onto it, you will probably destroy the threads. Old sprinkler heads are great tools for cleaning those threads up so that you can actually thread plastic on.

If that brass riser breaks out of the copper pipe, you have to dig and either solder it back in (very difficult for me), or cut the pipe and use Shark Bite copper fittings to patch in a new tee.

Also, that old adapter can travel up and down that riser, and you can easily adjust the sprinkler hight. There is about 2" of travel on my system. So you don't have to tighten the brass adapter on the brass riser(!), you can leave at wherever height you want. It's really a clever system actually, if you need to raise or lower heads, you just spin the sprinkler head. Just be careful if it's really hard to spin the head, because you will break the solder joint of that the brass riser if you use too much force. Learned that the hard way...probably should have tried to spray oil on the joint, instead I got a big pair of channel locks, broke that solder joint, and created a lot more work for myself!