will2sail
New Member
I had an 80 year old section of cast iron split down the side. I cut out the section and planned to simply install a section of 4" PVC with no-hub couplings. I read that the proper couplings were those wrapped with metal. I tried to do this, but the cast iron is 4.25" and the PVC is 4.5". When I tightened the clamps the metal buckled and the clamp slid sideways due to the difference in diameter. It did not appear I would get a good seal. I came up with the following solution and would welcome feedback.
I could have used longer rubber connectors and they would accommodate the different diameters, but I didn't want water sitting inside the lower coupling where the top of the cast iron was cut. This seemed like a leak waiting to happen and a place for any material to get caught and clog the pipe. I bought a section of 3" PVC and two 4" to 3" rubber adapter couplings and combined these with a short section of the 4" pipe and a 4" to 4" long rubber coupling. At the top I simply used the reducing coupling to connect the cast iron to 3" PVC, beveling the inner edge of the PVC and lowering it enough that the top is inside the smaller diameter of the coupling so there won't be a trap collecting water. At the bottom I left the 3" PVC long enough that it extends down inside the cast iron pipe an inch or more which ensures no water collecting here unless the pipe backs up. To seal the bottom I slipped a 4" to 3" coupling up the 3" pipe followed by a short section of 4" PVC pipe and one of the long rubber connectors. The bottom of the 4" PVC is sealed to the cast iron with the long coupling and the top to the 3" pipe with the 4" to 3" adapter. The bottom is held snugly in place by being inserted inside the cast iron pipe. The top is only supported by the rubber adapter. There should be no side load so this may be adequate but I am wondering if I should support the top of the pipe below the coupling with a strap or by foaming it into place.
I could have used longer rubber connectors and they would accommodate the different diameters, but I didn't want water sitting inside the lower coupling where the top of the cast iron was cut. This seemed like a leak waiting to happen and a place for any material to get caught and clog the pipe. I bought a section of 3" PVC and two 4" to 3" rubber adapter couplings and combined these with a short section of the 4" pipe and a 4" to 4" long rubber coupling. At the top I simply used the reducing coupling to connect the cast iron to 3" PVC, beveling the inner edge of the PVC and lowering it enough that the top is inside the smaller diameter of the coupling so there won't be a trap collecting water. At the bottom I left the 3" PVC long enough that it extends down inside the cast iron pipe an inch or more which ensures no water collecting here unless the pipe backs up. To seal the bottom I slipped a 4" to 3" coupling up the 3" pipe followed by a short section of 4" PVC pipe and one of the long rubber connectors. The bottom of the 4" PVC is sealed to the cast iron with the long coupling and the top to the 3" pipe with the 4" to 3" adapter. The bottom is held snugly in place by being inserted inside the cast iron pipe. The top is only supported by the rubber adapter. There should be no side load so this may be adequate but I am wondering if I should support the top of the pipe below the coupling with a strap or by foaming it into place.