Is it okay to use WD-40 inside some plumbing fixtures? Or would this cause leaks or other problems? I’m talking about the temperature control knob on my Delta Monitor 1700 series shower handle. This control knob is hard to move. I've taken the cover off the knob and I can see that there is a warning not to tighten the center screw too tight. That was not the problem. In fact, that screw had gotten so loose that the knob was about to come off. Would it work to spray some WD-40 into the area? Is there some other way to make the knob easier to move back and forth from hot to cold? My motivation for asking about this is that my wife finds it particularly difficult to adjust the water temperature. (I posted a similar question a few days ago under a different title, but I didn’t get any answers, so I’m trying a different title.)

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Is it stupid to use WD-40 inside plumbing fixture?

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In a word "Yes"

WD-40 is not a lubricant, it is a water displacement chemical. Some years ago a couple of guys working in the aircraft industry got a contract to develop a water displacement (removal) and rust inhibiter. Formula number 40 worked, hence the name WD-40. WD-40 is not a lubricant, repeated use will build up a sticky film on a surface.

So, I ask you. Is it logical to put a water displacement fluid inside a pluming fixture?

The problem is probably that the "O"s ring in the diverter are swollen and making it hard to turn. Many faucet manufacturers warrant their products for life. Moen does and I think Delta does also. Contact Delta for a replacement stem or a repair kit.

I would use Silicon Jel to lubricate the "O" rings. DO NOT use any petroleum based product such as oil or grease as they will deteriorate the neoprene. Good luck, Tom