Originally Posted by cave diver

Read my post #36 again. They don't crack from the torque of installing them correctly, in fact there is no force applied to the porcelain when done right. It is the lateral (sideways) torque that cracks them. This happens when your socket gets sideways as you attempt to tighten it down. This will break ANY plug and is a common mistake some people make.

Also, check the depth of your spark plug socket, it may be too shallow. To check, put a spark plug in the socket and see if the top of the plug comes up into the drive portion where the extension goes. If it does then when you are tightening the plug down you are putting direct pressure on the entire body of the plug and not on the 5/8" hex (Champion) or 9/16" hex (OEM Motorcrafts). This will also break a plug.

I've been turning wrenches on cars since the 60's and have seen and experienced a few things in that time.