Installing a 4" sewer cleanout

Installation of an ABS 4" two way cleanout tee in an existing cast iron sewer pipe NOTE: READ MIKE THOMAS' comment below. - As per Mike Thomas' comment below, the screw cap is inadvisable. It should be replaced with a loose fitting rubber cap instead (in case of sewer backup this cap will pop off and prevent the house filling with sewer water) Update: Note Richard's comments below (in reply to Mike Thomas' comment) : This idea might be fine in my situation where the clean out is OUTSIDE the house, clearly visible to anyone entering the house. However, you wouldn't want to implement this loose-cap idea if the clean out was in a basement or crawl-space - or indeed any place where a spillage might not be noticed for hours or days. Having poop belching out on to the lawn I can deal with... in a crawl space, not so much:)