Dave_s wrote:
I really need to remove, without damaging, my old brass impact
rainbird sprinklers from the 3/4 inch pipes they are mounted on.
Using plumbing wrenches and penetrating oil has not loosen them so
far. These sprinklers have been attached for the 50 years I owned
this house.
What other methods or materials can I use to loosen either the
sprinkler, or the coupler it is attached to?


Try a few cycles of tighten, loosen.

Or

Set up the wrench on the pipe with a good heavy block to stop it from moving at
all, with the wrench near the sprinkler. Apply maximum loosening pressure on the
wrench on the sprinkler, then hit it in the same direction with a hammer to
apply impact force.