I don't know about others saying buy a different size than you need but I purchased the 3" plug to replace my brass one and it worked flawlessly. I didn't feel comfortable installing another brass plug since the threads were somewhat damaged from having to destroy the original plug.

That said, having noticed some of the reviews with people over tightening this, I began by tightening the nut only until the threads caught on but was still loose. After that, I tightened the plug by hand until it was flush with the lip of the pipe, making sure that the threads hadn't jumped out of place due to over tightening. Next, I tighten the nut until it was snug, placed a bucket underneath and ran some water. It leaked as expected so I tightened it until it stopped. This may not be an option for everyone but it allowed me to tighten it only as much as I needed to.

I'll say, I'll never be afraid of doing this again as long as I have one of these on hand.

These are the only cleanouts I could find that would work in my super old XH (extra-heavy) cast iron whole house trap (Somerville). I had originally bought 2 used from Amazon Warehouse Deals which I regret since they were subtly damaged. I returned them and ended up buying new, which I installed with some slight modifications after guessing at the mistakes someone made installing the used ones. Here is a list of the modifications and then I will explain why I thought they were necessary.

Modifications:
- Removed the black rubber washer separating the carriage bolt from the bottom piece of ABS, then glued the carriage bolt in place. Frankly nothing sticks to ABS so I really don't know if it did anything but it appears to at least hold the bolt in place.
- Duck butter on the sloped inside of the top of the main rubber piece, thus allowing the top ABS piece to spin. Only on the very inside of the top. The bottom ABS has ridges and should stick to the bottom rubber. They come greased in that area on top (see pic) but it's not enough. In hindsight I could've just greased the bottom of the top ABS piece.
- Duck butter on the bottom of the white washer that sits between the top ABS piece and the nut, so that washer can spin independently of the ABS.
- Replaced nuts with some 3/8 galvanized ceiling plates (two dollars each at Home Depot) that I could turn like wing nuts, but heartier. I can use my fingers to tighten but I suspect that way it's much harder to overtighten and fail like the used ones I returned. I also like that once loose I can "yank" on the handles to remove the plugs.

Duck butter is basically plumber's lube that is non-oil based (since oils affect rubber). If you can't find it then use something non-oil based like a little bit of detergent or something. Also note removing the nuts is a little difficult because it seems like they very gently crimp the bolt end. Anyway if you're patient you can get them off, just turn slow and hold the carriage bolt head into the square hole so it doesn't strip.

Now why I thought all this was necessary. In the first four pics you can see one of the used ones that I ended up returning. I suspect in both cases someone overtightened the nut, probably with a ratchet, which eventually made the nut so tight it couldn't spin so then the bolt started to strip itself out of its square hole. I also suspect this is partially due to the design which puts a black rubber washer on the carriage bolt head, preventing the square part from fully entering the hole, which is why I removed it in my replacements.

In the last 3 pics you can see the replacements installed. You'll notice the rubber is flush with the top of the cleanout instead of the ABS plate being flush. That is because they only give you 1/2" to 5/8" of rubber, and my cleanouts are exactly that deep in threads before the pipe expands out. I installed by turning tight with my fingertips, just until the rubber started to bulge and then the top ABS piece and white plastic washer appeared to start moving just a little (that means it's pretty tight). I checked by trying to yank them out with all my might and they wouldn't budge.

Also while I'm at it I'll add one other thing, when you have a whole house trap and there's a clog, it's typically best to remove the plug on the street side and not the house side, because if it's street side you can reach in with gloves and unclog it without a ton of sewage that has backed up in the house drenching you. If you open the house side first usually sewage will barrel out if the trap is clogged. Of course if you live in an area where other people's sewage backs you up from the street side then that advice doesn't apply to you.

Well I know this is a long review, I'm not some yelp reviewer but hopefully this saves you some time because these are the ONLY cleanouts I can find that meet the dimensions of my old cast iron cleanouts (3 1/4" wide by 5/8" thread deep), and maybe yours too?

Update: Amazon changed the order of the pictures. Pics 1, 3, 4, 5 are of the used one that was damaged. Pics 2,6,7 are of the new installation.

A few minutes to install, and It worked like a charm on a very corroded cast iron clean-out flange. I used the Cheme Gripper Plug to replace a leaky cheap plastic plug & rubber gasket that a professional plumber had put in after he snaked-out my main drain line under the house. It sealed perfectly on a completely corroded clean-out flange. Like I usually do, I put plumber's (silicone) grease on the bolt threads, even though the nut and bolt are stainless steel. This makes sure that the nut can be unscrewed 10 years later in case calcium or other crud builds up on the threads. It also makes turning a nut (or screw) easier on installation.

Yes, as someone complained, the nut can't be completely unscrewed from the bolt. The manufacturer probably views this as a feature to ensure that you won't lose the nut in dark places.

Needed a 3 1/2 plug to replace an old cast iron plug. None of my local hardware stores carried 3.5 and I even tried to rig up one using a 3 and 4 in plug. Read the reviews for this one and decided to give it a try. Came pretty quick and fit perfectly with a nice seal. Probably won't last as long as the old cast iron (I'm guessing is at least 30 years old), but for $13, I can live with that.

This was a replacement for one ordered last week. The first one had the end threads damaged so the nut could not be removed. I wanted to replace the nut with a wingnut so I would not need a wrench. I returned it and now the one that came today had the same problem. Either there is a manufacturing issue o it is intentionally being done.