Finally sorted out that weird drain size in my 1950s vanity

I ran into the same nightmare with a 1948 bungalow kitchen sink last summer. The galvanized pipe looked like a wierd hybrid size becuase of the rust and layers of old lead paint. I wasted a whole Saturday trying to find a brass nut that woud catch those threads before I realized the pipe was just swollen from decades of internal corrosion.

I ended up using an Oatey transition coupling because I was worried about a standard rubber boot being too "squishy" under the sink where I might bump it with a cleaning bucket. Those old pipes are rarely a true 1-1/4 inch OD once you get the scale off. If you ever have to do this again, I found that spraying a bit of PB Blaster and letting it sit for twenty minutes helps that wire brush get down to the actual metal much faster. It's a filthy job, but beats cutting into the plaster and lath any day.

In my 1954 ranch, I had the opposite problem where the pipe seemed way too big for any standard kit. It turned out I was looking at a 1-1/2 inch iron pipe size (IPS) stub out, wich has an outside diameter of about 1.9 inches. Most people expect "one and a half" to mean the thin-walled tubular stuff you buy at a big box store, but that galvanized schedule 40 is a diffrent beast entirely. Instead of going the rubber coupling route, I used a brass trap adapter, often called a Marvel connector. I soaked the stub in PB Blaster for an hour and used a wire wheel on my drill to get the threads back to a usable state. Once the adapter was on, it let me transition directly to the modern PVC P-trap without having to worry about the rubber sleeve shifting or leaking behind the cabinet wall over time.

It is a real mess when the wall pipe is that thick-walled metal. If those threads are truly shot, I ussually reach for a Mission coupling. They have that stainless steel shield that keeps the connection from sagging, which I find stays a bit more secure than the standard unshielded Ferncos when you're dealing with the weight of a full trap.

Since you're dealing with those nasty galvanized threads, I always keep a tub of RectorSeal No. 5 in my bag. Even after a heavy wire brushing, those old pits can still leak like a sieve. Slathering that yellow RectorSeal on there is the only way I've found to get a permanent seal on 70-year-old steel. If the threads are too far gone and you have to use a boot, go with a Mission Rubber shielded coupling. The full stainless steel sleeve provides a much more rigid connection than a standard Fernco, wich is a lifesaver when the old pipe is pitted and not perfectly round anymore.

I'm a total newbie and just started looking under my own sink—I didn't even realize pipes could 'swell' from corrosion, that sounds terrifying. If I use one of those Fernco or Oatey couplings, do I need to worry about the pipe eventually rusting all the way through from the inside? My house is from the late 50s and I'm scared if I wire brush the threads too hard, the whole thing will just crumble. Is there a trick to knowing if the metal is still strong enough to hold a clamp?