Why are the spark plugs on a Corolla hard to get out seems like they might strip out of the head?

Answer 1

Electrolysis between the dissimilar metals of the aluminum head and the thread of the spark plug, has caused a corrosion which has bonded the two parts together. You should try a spray penetrant such as WD-40, but this may only be partially effective. Never-the-less, repeatedly soak the bases of the spark plugs. Put a socket with breaker bar and tap it repeatedly with a hammer, but only relatively light blows. Alternatively you can use a manual impact driver and impact socket, but again avoid forceful blows. Also refrain from using an air wrench. Hopefully, by repeating this process over time, you can work the spark plugs loose.

If you can't loosen up some of the plugs your alternatives are to leave the plugs as they are, take it to a shop, or continue to force the plugs loose. If you continue to force the plugs loose, you will likely end up stripping out the softer, aluminum threads in one or more of the spark plug holes. Fortunately they manufacture spark plug thread inserts that include a tap tool to provide an oversize threaded hole for the insert to thread into, to restore the hole. You can do this on your own. If you take it to a shop, the same thing will probably happen to them, and they will install the same thread insert or a helical insert which requires specialized tools and may be superior to the diy thread insert.

When/if you finally replace the spark plugs, apply a liberal coating of anti-seize or never-seize; two competing products that do the same thing. These compounds form a thin dielectric film between the two metals that inhibit electrolysis, and have a sacrificial metallic component that is acted upon by the harder metal instead of the aluminum cylinder head. Any other bolts that you install in the aluminum engine or other aluminum parts should be liberally coated with this compound.