So the faucet cartridge in Shower 1 was about 12 years old and really stiff. I replaced it with a new one. But then it started leaking through the middle of the new cartridge. I thought it was a faulty cartridge. So I tried another new one. There was still a leak. I then tried using the cartridge from Shower 2, which I knew worked fine, but it still leaked. I have tightened the locking nut as tightly as I dared lest I damage the threads, but it still leaks. Is there something wrong with the cartridge housing?
I remember when I initially removed the locking nut in Shower 1 that it was really stiff. Much stiffer then the locking nuts for the other faucets in my house. Did the original plumber screw it on too tight and damage the faucet housing? I do notice there are some aberrations on the inside of the housing in Shower 1. Please find some photos attached. Please also see these photos of the housing for Shower 2, which look a lot cleaner inside. Is this the problem? How do I fix it?
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Edit: forgot to mention, this is a mixer tap. here is an image with the handle on.
Edit (April 17): My next strategy is to cover up these holes at the side of the cartridge with plumbing tape. I will post an update about how this goes.
Edit 3: It didn't work. Water is still leaking from the cartridge. Guess its time to call a plumber :/