You will have to bust out the ceiling where the dripping is occuring to get to the pipes. Before doing so, check and see if your house was built with an access door for the valves. (Look on the other side of the wall where the valves are.) I have the very same problem and am going through ceiling replacement after repairing the pipes. It is a mess. Good luck.
Since two plumbers have tried and failed, I'm assuming that the problem is none of the obvious ones. I have two ideas for you, the second being the most effort, but also the most likely.
(1) If you have ceramic tile on the walls around the tub, sometimes cracks between the tiles will develop or the grout itself will develop leaks (become very porous). This would not necessarily show visible cracks, but could appear to be a drain problem since it will only leak during/after use. For this, you would need to patch the grout where needed and seal it well.
(2) You may have a leak in the piping under the tub. Since you mention a light fixture downstairs under it, it is likely that you can "create" access to the piping. This would require cutting a section of drywall out immediately under the drain and approaching the light fixture, about a foot wide should be enough to give you a decent look. Cut the drywall carefully so you can use the same piece to close the hole, with a little spackle, when you are done. Once open, turn on the water and watch to see where the leak is coming from. At that point you (or a plumber) should be able to identify what to do.
If your older house is "really" older and you have plaster instead of drywall, do the same thing, except you obviuously can't use the old plaster to fix the hole when you are done.
Yes the weight of someone in the tub plus the water can push the tub down far enough to cause the leak. Unscrew the flange in the tub that holds the drain to the tub. Get some plumbers putty and make a bead to fit between the tub and flange. Make sure the rubber gasket is clean ..use a bit of plumber's putty on this as well. Get an assistant to stand in the tub while you snug things up. Snug it up tight.... replace if the threads are stripped as you said. Plus if your house is less than a year old, I'd talk to the contractor/builder if possible about warranty work...most reputable builders will cover their work for the first year.
put the putty under the flange where it makes contact with the inside of the bath, push the flange to the bath & tighten nut underneath. Trim off excess. Have an overflow pipe [not intergral] make sure the top & bottom of the collar have rubber seals If the whole lot is plastic there may well be a leak down & along the thread of the waste.[it comes from the o/flow pipe collar] Silicone the thread and tighten nut. make sure it's dry first - silicone & water do not make a happy couple
Most flush valves are universal, but try to contact manufacturer and see if they can help, or try plumbing supply store (not a home depot) If you need further help, reach me via phone at https://www.6ya.com/expert/frankie_091f536560a54e12
Question edited for correct spelling of maker 'Noen' to 'Moen'.
'Fix' as in it is leaking? Or tighten it down because it is wobbly, or something completely different?
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It is leaking due to a defective mechanical seal. It needs to be replaced. To replace it, is to take the pumpcase off by taking off the bolts that hold it. Then, take the impeller off to take the mechanical seal behind it. Put the replacement in, and reassemble the pump. Just get in touch with us by e-mail to get more assistance.
Move the lever or turn the knob. Check the stop tap under the sink. Make sure the faucet control knob or lever is tight, There is normally a removable cover that hides the screw..
Question edited for clarity, full 'make model (what it is)'.
This a Menards special. To remove handles, look for a small hole hidden from the front. They take a small Allen Key.