broken spark plug

Probably a standard 5/8" size 3/8 drive socket wrench, and some oil.

Do you mean that the porcelain part broke off, or did the threaded base snap off and stay in the heads?

If you can still get on the hex base of the plug, then you do as the other poster has suggested: penetrating oil and a spark plug wrench. Nudge the spark plug base back and forth, adding oil, until you can get it to release. Go slowly and be very careful not to strip the threads out of the head.

When you finally get it out, be sure the inner tip and porcelain didnt fall into the cylinder. If it did, try to get it out before you start up the engine.

If you actually did snap off the threaded part from the hex base, it would be unusual. I have heard of this but have never seen or done it. The only way out would be to use an internal easyout, electrochemical machining, or suchlike.

In the words of Alice Cooper - Welcome to my nightmare....

Having been through this a few times I can tell you it isn't fun. If the shell broke and left the threaded sleeve in the head I have used a LARGE square style EZ out and HEAT to remove them. Don't use the twist style if at all possible, they wedge things tighter as you attempt to turn them. I first spray the area with carb cleaner to get any oil or grease out of the way. Then I take a torch (propane will work with MAPP or OX/Acet being better) and heat the sleeve up as hot as I can get it, red is nice. Then I wait a couple seconds for the heat to stabilize and let it start to cool. Now drive the EZ out in and spray some Break Free or other good penetrating oil on it, NOT WD 40. Now apply some pressure on the end of the EZ out and back the threaded sleeve out. I have only had one time that the sleeve didn't come out on the first try. I just drove the EZ out in again and it came out on the second try. I now have a custom made EZ out style item made from an older tapered reamer that works really nice because it has 8 gripping flukes on it. Once you have the sleeve out let everything cool down good, then use a spark plug thread chaser to clean out the threads. Don't skip this step unless you want the new plugs to do the same thing.