I'm installiing a new basement bathroom and when running a vent stack I would like to tap into the existing cast iron with PVC using a saddle on the cast. My question is what method is used to cut a hole in the cast iron, where I wish to place the saddle and connect the PVC.

If I were going to cut a hole in cast iron, (which I've never done,) I would have to go with a cutting torch. Any idea how hard cast iron is? I wore out three metal blades using a saws-all just sawing 2" cast iron pipe. Perhaps if you could locate a diamond tipped drill in the size you wish that would do it but for my money the easiest way to go is take a hitch around the vent stack and take up the slack with a jack. Then take a No-Hub drainage tee with a threaded branch and mark your top and bottom cuts. Make the bottom cut with a set of ratchet chain cutters and put a strain on the hitch while you make the top cut. You may now use No-Hub clamps and install the tee. Thread a PVC male adaptor into the branch and you're set to run with PVC. When installing the vent tee don't forget it goes in upside down for roof and rain drainage. However, let me warn you. If the stack that you wish to tap into has anything discharging above where you tap in you are forbidden to run the vent anyplace but out the roof or revent back into a existing vent at least 6" over the flood rim of the highest fixture that vents into it. More questions? Just click on back. Good luck. Tom

Hey, I am back. I have been able to read threads, but not post for almost a week. As usual, Tom's answers sounded as good or better than I could do.

In this case, you might try a metal cutting or grit edge hole saw if you have one. If it doesn't work, go rent the cutter like Tom said.

Tom,
Have you ever tried an angle grinder to cut cast iron??

No Tommy I haven't. ButI wonder how many grinder wheels you'd wear out getting through a length of 4" cast iron. Ya figure more wheels then the metal blades you'd wear out if you used a Saws-All?
When we have the room we use a set of cast iron snap cutters, SNAP!! and you're through. If we're in a tight space we use a set of ratchet chain cutters. Would you like to know how we had to cut cast iron pipe before there were snap and ratchet cutters?
I broke into plumbing back in the 40's as a plumbers helper,( go-fer and flunky) in my fathers shop in Beloit Wisconsin. When we roughed in a new house,( all lead and okum joints), the journeyman would call out measures and I would make a mark on the cast iron with a piece of soapstone. I would then proceed to take a cold chisel and a ball-peen hammer and mark up a line around the pipe. Iwould then follow that line, beating away with the chisel, untill the pipe broke in two. Ya couldn't smack it with too heavy a hammer because cast iron's brittle and will break. Coming out of the ground and doing upper floor drainage was even worse. From cast iron stacks we coverted to galvanized iron to pick up the various fixtures. 1 1/4" for lavatorys and 1 1/2" for sinks, laundry trays and tubs. We used ratchet threaders that you had to pump up and down. 1 1/2" was very hard to thread and the thread had to be at least a 1 1/2 to 1 3/4" deep. I weighed 120 pounds at 13 or 14 and didn't have the poop to pull the handle on the down, (cutting) stroke.
I had to jump up and lay on the handle and force it down by sheer body weight on every stroke. Plumbers today don't know how easy compared to what we went through back then. We had to breath in lead fumes from the lead pot as we melted down pig lead to pour joints. My lungs are still scared from breathing in fumes from the muriatic acid we used to boil out lime deposits in toilets and we used to protect our cast iron bathtubs with asbestos sheets pasted on with flour and water. This is my first post of the day and you'll have to pardon me if I got a bit chatty and took a stroll down "the faded yellow pages of yesterday". I just thought that someone might be interested in a bit of plumbing history. Ya'all have a good day! Tom

Speedball,
What do you mean when you say, put the tee in upside down for roof and rain drainage? The work will be done in the attic above the drains, I'm trying to avoid punching through the roof.

What you are planing is called a "revent". That's when you tie a vent back into a existing vent so you won't have to take it through the roof line. The fitting that connects the revent to the vent, (as a rule a drainage tee) will be inverted, (upside down) when connected. The reason being that when rain enters the vent the flow will go down, not up. It will come FROM the vent to the revent, not TO the vent from the revent. Inverting the tee will facilitate the flow into the revent and not hinder it. Besides, that's the way code dictates that it be done. Regards, Tom

Speedball,
That's great info on what plumbing was like... Sure sounds like hard work back in the day. Not sure how many grinder wheels it would take... I'm guessing the saws-all is the best bet... just curious.