Relacing Kohler 6N-2C Stem

I am replacing a cold water stem from Kohler and am not sure whether I am supposed to solder the top of the cartridge onto the body or if it stays lose with the washer on the top. One issue I am having is when I turn the handle I don't really get a hard stop, it just seems to keep tightening until I decide it is tight enough. Over time that causes the metal screw inside the stem cartridge to strip the plastic. Seems to me it should come to a hard stop when I turn the handle to turn the water off but I don't see how it would work with the stem I have purchased.

The part numer you mention is a Danco #. This is a generic, and you would always be better finding OEM parts.

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How do I find out what the OEM equivalent is for the stem I purchased?

stem

You do not solder ANYTHING. You do tighten the stem completely. If you do that then the barrel cannot rotate and you get a hard stop. Without that you get a drip because it is not tight and the barrel turns instead of the stem. You are not repairing it correctly.