How to seal these sewer clean outs?
A common approach is a rubber coupling (also called a &quot;Fernco®&quot; for the same reason adhesive bandages are called &quot;Band-Aids®&quot;)
Counter-intuitively, the last time I read the instructions carefully the ones with a stainless steel reinforcement (sheilded) were not recommended for burial and the plain rubber ones (unsheilded) (with stainless steel clamps) were, but read the instructions carefully yourself to be sure.
General approach is to cut off the iron pipe and use a rubber fitting to transition to PVC. Then be sure to protect the PVC from sunlight (cover it in some way, or at the least paint it. Sunlight is not good for PVC pipe.) You may need to use a wire brush and/or grinder to get the outside of the iron pipe smooth enough for a good seal.