Cast iron hub waste pipe connection

[Glen1]

I am new to the forum so forgive me if this was covered already.

I have a 4" Cast Iron hub that I need to convert to 3" PVC for a toilet installation. It was originally a lead elbow into a brass ring with an oakum joint. The lead was damaged and my mistake was not just cutting the lead and using a no-hub coupler on the brass section. Now that the CI is cleaned out of all the oakum I am trying to use a Fernco 4x3 donut gasket (extra wide) and a section of 3" PVC. I test fit the donut and it went in tightly with a hammer and I had to wrestle with it a little to get it back out. The PVC would not go in at all with the Fernco installed first. With the PVC inside the donut first, I can tap it into the CI until the very last section of donut which is slightly larger in diameter than the rest of the donut. Once this lip is inside of the CI I will have my seal. Fernco's website says to pound on it with a hammer and wedge but I don't want to potentially damage the donut, PVC, or anything in this 70 year-old system.

Does anyone have any advice or experience seating this properly? It is holding back leaks for now but since it is not fitted properly it won't stay that way forever. This is on my main stack so I have to complete it ASAP.

I included pictures of the hub and how far I was able to install the donut.

Thank you

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[Reach4]

https://www.fernco.com/downloads/installation-instructions/fernco-donut-installation says to lubricate with a soap solution.

 

[Sponsor]

 

[Cacher_Chick]

Use a file to take the edge off the PVC. I use straight dish soap to lube both sides and then do whatever fighting needs to be done. If you keep it straight it will go in. I have seen guys use a post and jack where there is something to push against, but it is very hard to know how much pressure is being applied that way, and your goal is always to do more good than harm.

 

[Terry]

Going into a 4" hub I prefer the 4" insert and a 4x3 flush bush, which I angle off, soap up and then tap in using a 2x4 flat against the bushing and a hammer.

 

[Glen1]

Thanks for the advice. It took more time and effort than I care to admit, but once I got a small part of that lip inside the CI the rest of the rubber went in with significantly less effort. This thing is in there rock solid. I wish I would have seen Terry's idea first, it looks a bit easier than what I went thru. Bottom line, it doesn't leak!