I help take care of some property in south Florida that has a well for irrigation and both flush-buried popups and stick riser mounted fixed heads. We get a lot of careless damage from school children in addition to damage from well grit. Orbit, Hunter and Rain Bird popups use interchangeable tips. The short one shown above is an Orbit and the taller one a Hunter. The filters are also interchangeable and easily cleaned with an old toothbrush.
To save some money and trips to Home Depot, I have been reusing the core risers on popups to make fixed heads for watering the hedges.
Step 1: Save the Riser Core and Cut It
Unscrew the top cap on the pop-up and pull out the core. The spring is retained by the tip that we are going to reuse, so unscrew the tip and discard the spring and shell of the pop-up. There is a nylon filter just under the tip that needs to be removed and cleaned. An old toothbrush works well for cleaning. Put the filter back in and screw the tip back on.
The bottom of the core has a part that retains the bottom of the spring. Cut that off with a hacksaw, but the overall length of the piece you have left should be at least 3 inches because of what comes next.
Step 2: Choose a Base for an Adapter
We use standard 1/2" pipe risers in the hedges, so I have several 1/2" pvc adapters that have either male or female threads on one end and a glue socket on the other. These cost less than $1 each. The riser core I cut off is not pvc and it isn't really a tight fit for the glue socket, either, so I heat the cut end with a propane torch or candle or hair dryer JUST ENOUGH TO SOFTEN IT. If you use a torch to heat the end you have to be very careful or you will destroy it.
While the end is still warm and soft, shove it down hard into the glue socket and twist a little and wait for it to cool, then pull it back out. Don't worry too much what the smooshed, twisted end looks like as long as it fits tight. Don't use glue in this step. You are heating and deforming the riser end to make it a better for the pvc. Final assembly is the next step. If you are using a propane torch turn it off before final assembly. The glue is very flammable.
Step 3: Glue the Parts and Wait
You should wear nitrile gloves for this unless you don't mind wearing pvc glue that drips off the part.
I use the brand of PVC glue shown above. Blue pvc cement has body than clear cement and doesn't shrink as much when it dries. The "red hot" version has an aggressive solvent that does not require cleaning and priming. Use a generous amount of glue to both parts, but be careful gluing the female adapter because excess glue will drip down the inside and get on the threads. Twist the parts together and then go back around the top rim with glue to insure the seam is filled.
Set the piece aside upright for an hour to dry. PVC cement was not intended to glue black vinyl pipe but it works if you use enough glue to fill any voids .