Re: New Shower Head not fitting into existing shower stem
This seems odd to me, I've never known shower necks to have different thread sizes, they're usually 1/2" pipe thread on both ends. There may be an adapter to go from the standard thread to what you need. Best bet is to take your existing shower neck and the new shower head into the store to match up a replacement.
Shower necks are usually pretty easy to remove. Wrap a rag around the neck and then use pliers or a pipe wrench, turning counter-clockwise, to remove the neck. It should be just that simple. On older plumbing or improperly installed plumbing, it's possible to break the neck or the fitting inside the wall, but generally this isn't an issue. When you go to install the new neck, wrap the threads with teflon tape (about 10 wraps ) and then a little thread sealant (Rector-Seal is the best ). The new shower head should have a rubber washer to seal it on the neck so the threads there won't need any teflon tape or sealant.
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