If you want to fit a 2” PVC pipe with 2.375” OD in a non load bearing wall stud, it needs to be less than or equal to 60% of the width of the stud. i.e a stud which would need to be just over 3.95” in width. If you framed a wall 2x4 wall with thickness 3.5” you can’t fit the pipe by code.
Could you add a 1x2 to give yourself another 3/4” thickness and meet code ? Will this actually stabilize the wall?
Ask your inspection office whether stud shoes are acceptable. They're sometimes listed for code compliance. Quick and easy, if not terribly cheap.
I recently moved a washing machine connection, and had to deal with 2x4 walls. I had to run the pipe through several studs, and it appeared to be load bearing so I didn't want to take any chances at all. Reframing the wall was out of the question for this projects scope, and that would have also destroyed a wall in the bathroom on the other side.
What I ended up doing was basically building another 2x4 wall in front of the existing wall and ran the pipes through those "studs". That wall was not structural whatsoever, so I didn't worry about the large holes. I was able to attach the new 2x4s to the old ones in several places.
Because of the large holes in the 2x4s, I did use nail protector plates over each hole to avoid accidents when the new drywall was hung.
1x2's are typically very low quality and I wouldn't trust them to add any stiffness to the wall. On a non-load bearing wall you might be able to get away with adding 2x2s, but I'm not even sure if I would do that. A 2" PVC pipe requires a 2 3/8" hole (I think?) and that is a shockingly big chunk of the 2x4 once you see it in person. I thought it would be better looking at it on paper, but its a very large hole.
The basis of the code problem with a big hole for a big pipe through a narrow stud is that removing too much material weakens the stud.
If you attach a 1x2 to give yourself a little more width, you have created your own home brewed engineered lumber extended wall stud. Technically that won't satisfy the code as a structural member unless you had an engineer evaluate it and OK it as a replacement for the studs and OK that it's strong enough with a big hole in it.
So unfortunately reframing the wall with 2x6s is probably the only practical option.
No, tacking on a wood strip does not make the framing member any wider. There is no Law that requires pipes to be run through holes in framing members, or to be run inside walls at all.
