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I have a PVC pipe that supplies water to a valve, and then goes to my toilet. It's approx 1/2" inner diameter, and 5/8" outer diameter; it's stamped on the outside of the pipe that it is 1/2". The standard run of the mill PVC 1/2" that is "Schedule 40" is about 0.8" outer diameter, but is also stamped 1/2"

The valve fitting seems to fit the 5/8" outer diameter one and the valve is labeled for 1/8" nom., and of course then would not fit standard Schedule 40 1/2". Can someone clarify the difference? Is there some sort of PVC pipe type I do not know of?

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There is PVC and CPVC - pvc is white and IIRC usually slightly thicker than CPVC. CPVC is more of a beige color. – The Evil Greebo Jun 21 '12 at 12:53
There is thin wall PVC pipe (schedule 20?) But it's OD matches sch 40, not the ID. I have also never seen it used for a pressure line, only for DWV (usually only tail pieces from a sink drain). – Dave Nay Jun 21 '12 at 13:25
It is very important to remember that if the ID is larger than the OD, it is not pipe...(;-)...sorry, couldn't resist. – lqlarry Jun 22 '12 at 1:52

It's not PVC, it's either PB or PEX, which are sized similar to copper tubing. They normally are connected with barbed fittings with compression rings but compression fittings with metal inserts also can be used.

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