Do I need to cut the drywall to open this basement cleanout?

You definitly need to cut that drywall away before you even think about putting a wrench on it. If you try to force a pipe wrench into a tight, jagged hole, you’re just going to round off the edges of the nut or crack the surrounding plaster. You want about a 4-inch to 5-inch opening around the plug so you have enought swing room for your tool. It’s better to make a clean, intentional hole now than to deal with a crumbling mess once you start cranking on that metal. If you have a hole saw, that’s the neatest way to do it. Just center it on the bolt and let it rip. One trick I’ve used on these old basement cleanouts is drilling a small 1/4-inch pilot hole right thru the center of the plug before unscrewing it. If that line is backed up all the way to the stack, there’s a lot of head pressure behind that plug. Drilling a small hole lets the sewage drain out slowly into a bucket rather than blasting you in the face when the threads finally let go.

Once you get the plug out, don’t just shove the snake cable in there. Those rental snakes are heavy-duty and will absolutely eat the threads off your cleanout pipe if you aren't careful. Pick up a cheap **PVC male adapter** and thread it in by hand first. It acts as a guide and protects the cast iron or brass threads from the vibrating cable. It’ll save you a massive headache when you try to seal everything back up later. Since the wall is already finished, I wouldn't put that old square-head plug back in. Buy a recessed **flush plug** and a stainless steel cover plate. The plug sits inside the pipe, and the plate screws into it to cover the hole in the drywall. It looks a lot more proffesional than a bolt sticking out of the wall and makes future access much easier.

That pilot hole idea sounds like a lifesaver, but what happens if I drill it and then the main plug is still totally stuck? I’d be worried about having a permanent leak I can’t stop. Is there a specific type of drill bit that works best for metal plugs like that? My drill is just a basic cordless one. Also, for the PVC adapter, do I need to use any of that white tape on the threads while I'm snaking it, or is just hand-tightening enought to keep the mess contained?

If you’re worried about that plug being seized, soak it in PB Blaster for 24 hours before you touch it. Give the square head a few solid raps with a hammer to shock the threads loose. If it's brass, be careful—it's easy to round off. Use a big pipe wrench and, if you need more torque, slide a piece of pipe over the handle for leverage. Just don't go so crazy that you crack the main stack. If it realy won't budge after that, you might have to carefully notch it with a hacksaw blade and chip it out, but that's a last resort.