According to Richard J Kinch :
Steve B writes:

Are these things rebuildable? Are they worth it to rebuild?

Maybe, and maybe. If you can do it without hand-excavating.

All of the units I'm familiar with will unscrew in such a way
to leave only the exterior housing in the ground.

Once out, you'll want to go over the mechanicals with a toothbrush
and some partially diluted CLR or similar rust/scale remover,
clean strainers and output ports, and check for wear. Check
the springs on impact sprinklers, and make sure that the pivot
points are clean/moving freely.

Another thing to check is water pressure. Inground systems
(especially impact sprinklers) won't work right unless the
pressure is high enough. On a well, best results sometimes
require you to tweak the set point on the pump pressure switch
up a bit.

[I carefully designed mine to work with heads spec'd at lower
pressures. Still, it took raising the set point to 45PSI
before it worked to my satisfaction.]

Check also for leaks.

If, after cleaning and ensuring your water pressure is high
enough, and they still don't work well (or you determine the
old one is worn out), go buy one identical sprinkler, and swap
the guts into the existing housing and see if it works any
better.

If it doesn't, you should step back and double check the design
of the system in terms of flow rates and pressures. Or consider
changing sprinklers.

Rainbird is well regarded in the trade. Orbit seems so too (besides,
Orbit bought out HD's non-Orbit stuff, so HD only carries Orbit. At
least in Canada...) Orbit's gear drive (eg: Voyager II line) are
nice and do seem to live a long time, and Orbit's service department
is _very_ good. I'm having less luck with impact heads.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.