The original HR-1 and HR-1.5 Hydrorain and Hardie valves had a flat-sided solenoid. This round solenoid replaces that flat-sided solenoid on all versions of the Hydrorain or Hardie HR-1 solenoid valves. It also fits the one and one-half inch Hydrorain valves.

This solenoid is a direct replacement, but also be aware that most valve problems are caused by the Diaphragm, not by the Solenoid. Your valve actually opens and close hydraulically, with the solenoid acting as the electrical switch that allows the release of water to enable the valve to operate hydraulically. Solenoids are little more than an electro-magnet and seldom go bad. Not to say you may not have a bad solenoid, but the odds are that if you have a problem with a valve, the cause will be the diaphragm. 

I usually liken blaming the solenoid for a valve problem to blaming the light switch when a bulb blows.