The cleanout must be kept accessible. I think your options are limited to the following (but I may be wrong):

You could extend the cast iron at the cleanout by removing the cleanout fitting and putting in a new cast iron wye fitting, such that new pvc ties to the branch of the wye and the cleanout is reinstalled in straight portion of the wye.

However, you might find that it is better to cut the existing cast iron line and put a new fitting in to accept the new bath drain line.

The new drain line might also need a cleanout where it turns from vertical to horizontal.

The cleanout is not IPT and is NOT designed to have pipe attached.
Add a new sitting, and (as noted above) make sure you have the correct clean outs.
They are REQUIRED, not optional.

OK,

Well it took me awhile to get back to here, but let me mention what I ended up doing. (Tell me if my city inspector is going to be upset or not!)

I used a Fernco donut and put this on the inside of the cleanout. Then I put a 3" PVC SCH40 pipe into the donut. I should mention that this was EXTREMELY hard!! The seal is definitely airtight.

The cleaout was a verticle one, so I put a 3" Wye on top of the pipe and a cleanout access on top of this. So the cleanout is effectively 1 foot higher than it used to be.

Then on the other side of the Wye I have a 1/4 street fitting and that's where my new waste line from upstairs comes from. That was it! What do you think, was it an OK solution?

Would an inspector have a problem with this? Like I said, this is air tight, and I had someone dump 5 - 10 gallons of water down the drain upstairs while I watched it at the cast-iron tie-in... No leaks anywhere.

Thanks,
Shade

Contrary to what was stated above the thread is an NPT (National Pipe Taper) thread.

For pressure piping they do make 2-1/2" and 3-1/2" pipe and it is connected by means of NPT threaded fittings however we are not permitted to use pressure fittings on DWV(drain, waste & vent) piping.

DWV grade cast iron and copper pipe is made in 1-1/4", 1-1/2", 2", 2-1/2"(very rarely seen today), 3" and 4".

The International Residential Code table IRC 3005.2 and the Uniform Plumbing Code table UPC 707 list the minimum size of cleanouts required for each size of DWV pipe. For a 3" pipe we are permitted to use a 2-1/2" cleanout and for 4" or larger pipe the minimum size of cleanout permitted is 3-1/2".

There are only two instances when threads joints are permitted on a DWV system.

1. We may use an ABS Female thread adapter and a PVC male thread adapter to make a transition from ABS to PVC. We may not use a PVC Female thread adapter because the codes totally prohibit the use of PVC Female Thread Adapters.

2. Cleanout caps may be attached by means of an NPT thread.

Understanding that there are no male threaded DWV grade fittings and further understanding that there are no 3-1/2" DWV fittings it then stands that we commonly use a 3-1/2" cleanout as the main cleanout on a stack in an effort to hamper any attempts to defeat or modify the use of the cleanout.

The prevous post states that the 3_1/2" cleanout was first reduced to 3" then a Wye was attached to permit adding the new pipes.

"Would an inspector have a problem with this? "

In most jurisdictions the inspector would have no problem at all with this. He or she will simply hang a red tag on it stating the installation is code prohibited.

"Contrary to what was stated above the thread is an NPT (National Pipe Taper) thread."

Might be now, but I have worked on plenty of old ones that would not take a tap to clean up corroded threads.
Ticked me off since I had to get another tap and they are not cheap.

According to the experts at the Pipe Mill here in Sharon, Pa they have been using the same 2-1/2" and 3-1/2"NPT threads since 1896