I'm repairing a toilet flang on an 80-year-old home with old cast iron plumbing. What I have is a 4.5 inch inside diameter cast iron hub. A 4 inch ID PVC pipe drops in nice and snug, but I cant figure out a good way to create a seal.

Hub pictured here:

4.5 inch cast iron hub

This next picture shows you what the previous owner did. They just droped 4 inch ABS in and put some kind of glue or plumbers putty around the edge. It obviously did not create a good seal.

4 inch ABS

A few places on line have suggested using a compression donut such as the one below. However, the 4 inch compression donut is too large to fit in the hub and the 3 inch compression donut is too small.

Compression donut

What other options do I have? I am at a loss at this point. Some other suggestions have been a rubber sleeve, but I have not found one that would fit.

  • 1
    Picture of the donut not fitting? These don't go in easy. It's actually decidedly difficult. I did cast iron once. You have to keep it clean, straight, lubed, and push until you see stars.
    – Mazura
    Commented Feb 23, 2021 at 0:37

1 Answer 1


There seems to be a 3/16" thick layer of crud on the inside surface of the hub. Scrape that out and then try the 4" "donut"

  • If it's actually a hub and not just half of what's left of a flange, OP just ain't pushing hard enough, +1. Rubber mallet and lube; cast iron is a bitch. If it is half of a flange, take it back down to the next hub.
    – Mazura
    Commented Feb 22, 2021 at 16:53
  • 1
    Jasen, sometimes you just need a second set of eyes. As soon as you pointed this out I could see it in the picture. Sure enough, took my metal putty knife to it and it flaked right off. ID of the hub is actually 4 7/8 once you get rid of the crud.
    – ferics2
    Commented Feb 23, 2021 at 4:57

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