I need some help with a leak. I just replaced the cast iron headers on my Raypak 336A heater. I threaded the CPVC couplers directly into the cast iron flange as the install manual instructed. I tightened them as tight as I could but once I turned the pump on I have a leak from the threads between the cast iron flange and the CPVC couplers. I'm thinking I should have put some sort of sealant on the threads even though the manual didn't instruct that. Anyone know what I should use on the threads so they seal? Picture of the grey CPVC couplers below. 
