When dealing with a car with aluminum head(s), what is the proper way to extract a broken sparkplug?
Before doing anything else, get some penetrating oil. Reducing friction in the threads will dramatically reduce the amount of twisting force required to remove the stuck plug, making the job way easier & reducing the chance of making things worse (i.e. breaking off a screw extractor!)
Follow the instructions on the container, or just spray around the threads, and let it sit. Tapping with a screwdriver or wrench will help it penetrate and break up oxidation.
A couple ways to go about this, depending on the exact situation:
If the ceramic stem is broken, and you can still fit a deep socket on it, go with that. If the nut itself is stripped so much that you can't get a socket or an open-end wrench on it, or if the nut is broken, you should try using pliers to remove the spark plug. You could also try something like a deep socket Craftsman Bolt-Out bolt remover if it is stripped. Another option would be to drill a hole in from the top, going straight down, and using a screw extractor to remove it.
Pictures to help: Craftsman Bolt Out:
One more option:
If you tried taking the spark plug out of a hot/warm engine and the plug broke off between the
Take a careful look at this special situation before you go the extractor route. An extractor in inexperienced hands can ruin a head.
I'm sorry I don't have pictures.