I'm not sure actually. I have the same situation on my kalk reactor, ie. a MJ600. The original crapped out and I wanted to replace it with a MJ12 because I had a couple spares of those. But I wasn't able to get the cap (that's attached to my reactor) to fit properly on the 12. I went and bought another 600 and it fit. Sooooo... hopefully it works for you, but ... it didn't for me
