Well I need to replace the clay sewer pipe that connects my home office to our house sewer line. It is all clay pipe. I am finding root entry at every joint, and they are 3' joints. This was layed in 1940.
What is the best way to cut this clay pipe, and connect PVC to it. I'll be replacing from the toilet under my office, to the main line, and installing a cleanout.

You'r better off replacing it completely with that green PVC sewer pipe. Clay is old stuff and will allways need repairing. That 4in PVC sewer pipe is cheap and easy to lay. Make sure you lay it at an incline for drainage. If you have to lay it over a far distance install a "T" with a cleanout every 80 foot or so. I like the system I had because I could drain my RV tanks into it.

To cut clay pipe you need a special tool don't remember what its called, but replce it all you'll be way ahead.

Replace it with pvc, like said above.

you can rent a chain cutter or cut it with a grinder and masonry wheel. a rubber boot is used to connect the two together.

Ok So a chain cutter will cut it straight, like it would on cast iron sewer line? I got a dianond cutting blade, but my grinder quit.

as long as the clay pipe is not totally rotten it works pretty well, there is the chance it could crush if it is in too bad of shape.

cutting is the easy part just break it and then get yourself a cressent wrench open it enough to fit the clay pipe and holding it backwards tap it with your hand and go all the way around till it breaks smooth as you want it. 3" PVC is what I would use not 4". Rubber boots to attach is all you need. I have a plumbing business so if you need any more advice just holloar.

buy a clay to cast (pvc) ct adapter...nothing more than a rubber boot with hose clamps to adapt down the regular pvc size...the clay is much larger. As stated before, I think code is 75' max and then you must have cleanout.

If the pipe has been there all that time, most likely when you expose it to air it will crack a zillon ways.Like said above, get a CT adaptor clay to pvc and try to go to a spigot end on the clay pipe, on the other end rent a quickie saw with a fiber blade masonary and connect with CT adpt. Last I layed in FT. Worth, a cleanout was required atleast every 90' and no more.For extra good measures get a couple of bags of sand mix and pour around the CT adpt's. Its getting harder to find these adpt's and clay pipe but there used to be a place on Irving blvd. called US Filter that was a pipe supplier.It's been 7yrs since I been there but who knows it mite still be in operation. Good Luck
After re-reading your post, I noticed you are going back to the main with this. Most likely you will have to test the line you layed and only a licenced plumber can make the connection at the main. Better talk with the city and see what your op's are.

In Texas a HomeOwners whose house is Homesteaded can make any repair a Licensed Plumber can make. Water, Gas, Sewer.....

If it were me I would take all the clay pipe out and put in PVC or Poly line

Connect at city end with a "ClampALL" connection, I would not use a cheap CT boot or CT adapter

http://www.clampall.com/performance.htm

Not sayin thats not true, but i'd have to see that in writing about being able to make connection at main in city ROW.

RR, is a licensed plumber in Texas...

I respect that, but making the actuall tie in on the city main is a no no.From the easement line to the house yes the home owner can do as he like's but the tie in on main must be made by a lic; plumber and in some cities here in Texas even a lic; plumber cant make it. Sewer, gas, water.I know I would want the city to make the tie in or a plumber simply because if the service line is clay then most likely the main is also.Old clay pipe is tricky, been thar done that.

Do yourself a favor and lay some tracer wire alongside the pipe before you cover it up so you can find it with a metal detector next time.