Whether or not to remove and replace an existing cast iron closet flange is really an on the spot assessement and judgement call that is based upon a number of factors.
Cast iron is really a very rugged metal even though it often does visually appear to be in bad shape. Quite often when you first lift the toilet the flange appears much worse than it really is.
Begin by using a stiff putty knife and wire brush to remove as much of the old wax, rust and scale off the top of the flange as you can. Once it is cleaned off normally you will see that the basic part of the flange is still in good shape, although it is common for the thin metal rim around the bolt slots to be cracked, broken or even missing.
Most hardware stores only carry the little 4 inch reinforcing plates and like you, i am not a big proponent of those.
A much better solution is to install a repair flange, which is a two piece complete ring flange plate that fits directly under the existing flange and wraps completely around the pipe. After the two pieces are fit in place, they are bolted together to form a new flange plate ring. (Complete repair flanges are not common in the hardware or big box home supply store and you will probably need to go to a plumbing supply house to find them.)
Removing an existing flange without damaging the soil pipe can be a real challenge. It would really depend upon the accessibility, but often cutting the pipe and replacing the compete closet bend will prove to be a much simpler task, however, cutting a 3" or 4" cast iron pipe can also be a challenge.
Cast iron soil pipe can be cut with a reciprocating saw (sawzall), but in most cases it will be very slow going and require repeatedly changing blades. In this application cheap blades will prove nearly useless, and even when using high quality Lexox Cast Iron blades you must expect to go through 3 to 4 blades before you finish the cut. The end result will be about $10 to $15 in blades and 30 to 40 minutes to make the cut.
An abrasive type metal cutting blade on a circular saw can cut the cast iron pipe but in most instances you will not have enough clearance to use that type of saw.
A secton of the cast iron pipe can be easily broken out with two hammers (See illustration in the forum "illustration section") Then a new section of PVC can be installed by using a Fernco Donut gasket or oakum and epoxy.
The preferred method is to cut the existing cast iron pipe with a "Cast Iron Pipe Snap Cutter". Snap cutters are a relatively expensive tool that fits around the pipe like a chain wrench, then as the handle is tightened the chain has cutters that cause a clean fracture line around the pipe.
Snap cutters can usually be rented at your local "Tool Rental Company" for about $15 to $20 a day.
The snap cutter is easy to set up and use and will normally complete the cut in under 5 minutes.
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