How can I successfully replace a plastic commode flange?

Pianoman...

If there is enough room between old flange and toilet elbow to cut the pipe, add a coupling and a closet flange (or street closet flange) then you can use an inside pipe cutter (see pic. #1) and cut down inside the pipe, add coupling, measure pipe, cut pipe and install pipe and flange to correct height (flange should SIT tight down on top of finished floor).

Be careful in assessing whether you can install the fittings... if looking inside the pipe between old flange and elbow it may look like you have plenty of room... but remember that 3" pipe and fittings have an 1.5" socket fitting depth (or make up depth for street fittings)... hope that makes sense.

If that is not an option, then may be able to install extension repair flanges (see pic. #2)... these are usually sold as a kit which come with repair flanges of different thicknesses (flat pieces in pic... not the white flange itself). And to really drive it home... I added an attachment for you to read about installation (click on closetflange2.pdf at bottom left).

Here, clean off old gasket real good, remove screws from old flange and then install repair flanges right over the old flange (to at least FLUSH with floor or slightly higher), secure with rustproof screws, then install a reinforced wax gasket (has neoprene inside), 5/16" johhni bolts and you should have a good strong installation.

Some of these kits come with gaskets on them (as in the pic.)... some need to have silicone installed between the flanges (let silicone dry before setting/testing toilet).

The best option, of course, is to replace entire flange... but if can't.. these repair flanges work great, too!

Good luck with it... Mark

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Piano. Is your old flange back or white and what exactly is wrong with it?