We're going to show you how to properly gap and replace your spark plugs. While this seems like a trivial thing to do, many hobbyists do it incorrectly. Some don't know how to gap a plug and others don't know how to tighten them in the engine's head.
The reason the plug has to be gapped to a given specification is that the secondary ignition system (the coil and spark plug wires) on your car was designed to provide a certain amount of voltage to the plugs. That voltage has to produce a consistent spark across the plug's electrode under the harsh conditions inside the combustion chamber. Too little a gap and the spark won't necessarily ignite the fuel/air mixture efficiently, in addition to contributing to the tendency toward fouling (debris accumulates between the electrode and ground, shorting out the plug.) Too great a gap will cause missing under engine load and high speed running, in addition to erosion of the electrode.
Here are the two types of gauges used for gapping plugs, the feeler gauge on the left and wire gauge on the right.
Gapping should be done with a wire gauge, but a regular feeler gauge will suffice. The reason the wire gauge works better is that it prevents the tendency toward bending the plug's electrode at an angle. Doing so causes an incorrect gap, so when using a flat feeler gauge, just be sure the electrode is lined up as parallel with the center electrode as possible.
Wire gauges allow precise gaps because it's impossible to put it in at the wrong angle.
Slide the proper thickness gauge between the center and side electrodes. If the gap is too narrow, gently pry up the side electrode. If too wide, gently tap on it with a screwdriver handle until its gap is just wide enough to slide the gauge through while feeling resistance. Don't use pliers to bend the electrode. It's too easy to break one.
Feeler gauges are precise only when they are parallel to the surface being gapped.
All the plugs gapped? Okay, now for the installation. We assume you've taken out the old plugs and made sure the threaded holes in the cylinder head are clean. If not, take a wire brush or dental pick and clean out the threads, then stick a vacuum cleaner nozzle at the orifice to suck out any larger pieces of crud.
Take each plug and start it into the threaded opening with your fingers to avoid cross-threading. Once the plug is turned in the threads a few times, put the socket on it and tighten until snug.
Golden Rule of Spark Plugs: The plugs don't hold the engine together, so don't tighten them as if they do!
Plugs only need to be tightened to about 10 lb-ft of torque. You can do this without a torque wrench if you use a ratchet handle about a foot long and imagine hanging a 10-lb weight on the end. That's how much force you should apply.
Once all the plugs are in, re-attach the wires and make sure they are secure.
— Keep the plug's threads clean, as well as the threads in the cylinder head
— Mark the plug wires in some way so that you know the proper firing order
— Gap correctly, being careful not to break off the electrode
— Remove plug wires by twisting as you pull. This helps prevent breaking off the insulator.
— Follow the route of each wire from the distributor cap to the plug and verify that the proper firing order has been maintained.
— Overtighten plugs.
— Forget to use the plug's crush-washer if a particular engine requires one.
— Assume everything's working right. Start up the engine and make sure.
— Allow plug wires to tough exhaust components.
These days most cars use platinum-tipped (or other fancy metals) plugs. These plugs are designed to fire under the high-voltage conditions generated by high-energy ignition systems. They work fine in new cars but, due to very high internal resistance, probably won't function in your old, points-type ignition. Don't waste your money on them.
"What about plugs in the aluminum heads? I use a drop of oil on the threads, and make sure it's cool before attempting to remove plugs."
SCG: Good advice. You can also use silicone lubricant rather than oil...