OATEY 268038 Cherne® 3" Iron Grip Bypass Plug, 11 PSI

Maximum back pressure is 11 psi (0.76 bar), which works out to 25 feet (7.6 m) of water column. That's the ceiling for a standard DWV riser test on a three story stack; if your column of water will stand taller than 25 feet above the plug, you're over the rating and need a higher pressure plug.

The bypass is 1/2 inch male thread. You can thread on a hose adapter, a gauge, a shutoff, or a nipple and run flow through the plug while it's still seated. The main reason plumbers pick a bypass plug over a solid one is so the line can be drained or pressure relieved before the plug is pulled, instead of taking the full head of water in the face.

The Iron Grip is a cast iron mechanical plug, tightened with a wrench to expand a gasket against the pipe wall. It holds without air, so there's nothing to leak down overnight, and it can stay in the line permanently if you want it to. An inflatable test ball goes in faster and passes through a smaller access opening, but it needs an air source and will lose pressure over a long hold. For a test that sits for days, or a plug you intend to leave in place, the mechanical plug is the one.

Yes. Cherne rates the Iron Grip as either a temporary or permanent plug, and the cast iron body is what makes that possible; it won't degrade sitting in a buried or capped line the way a plastic or rubber-bodied test plug can. Plenty of contractors set one in a cleanout or a future connection stub and simply leave it.