you said..
Initial results were frigtening: the pump didn't work anymore. It would make noise and seemed to be spinning, but it couldn't move water more than about three inches vertically. I could blow in the end of the output hose and push air right through the pump with ease. What surprised me was what happened when the water flowed back into the submerged housing: the pump began pumping. Once it had cleared all of the air, it was running like new, and maybe a little quieter because of the mod. I tried it many times, and concluded that I could only get the pump going by letting it spin up full of air and then suck in water.
Thats EXACTLY what happened when I tried it with my mag3. It wouldnt spin up unless it was started without water inside it. I used JB Weld to lock up the impeller/magnet. I was fastidious in making sure I didnt glue them together off balance ( at least to a visual inspection anyway) I tried running it in a bucket with the front part off to see what was happening, and it would spin when it was plugged in but not very fast. I had to scrape all that crap off to separate the two pieces to get it to work again
. Once I quit screwing with it and just plugged it in it worked fine. After my JB weld experience, strangely enough the minor clattering that I was attempting to cure went away. Could be they just need some time to wear in? BTW mine has never leaked from the front as others have reported. Thanks for the excellent documentation! I'm quite pleased with my Mag 3. I got mine from reefgeek as well, and also ordered two spare impellers. My experience with reekgeek was great, I would order from them again.