Every year I have to replace one or two of these. Doesn't speak well for the longevity of the product, but thankfully the solenoids are cheaper than replacing the entire valve.
Had one badly cracked one leaking and several with lesser cracks. The system was probably 20 years old so Replaces all 6 selenoids. No more leaks and good to go for another 20 years.
On the newer Irritrol Valves, the white retaining cap on the solenoid needs to be removed with a set of needle-nosed pliers or something similar. Otherwise, the threads on the solenoid will strip because it bottoms out on the valve body.