My neighbor has a 1992 Mercury Grand Marquis and asked if I could change the spark plugs. I removed the spark plug on the passenger side closest to the radiator and it looks good (aged but no issues). The plugs sit deep in a well and appears to be an aluminum block. Tried to remove the next plug but it won't move. Afraid to use too much force and damage the threads. Any suggestions to safely replace the plugs or is this a job for a mechanic shop?

 

Without the proper tools and equipment if you do strip out the threads, I'd recommend you take it in - either to a Ford dealer or a shop that has a lot of experience with Ford Triton spark plug changes. Yes, that is an aluminum head, and it only has 4 threads for the spark plugs. They are actually not known for sticking, they are known for loosening and blowing out due to an insufficient factory torque spec. If the threads do strip out, they can be repaired with inserts without pulling the head - but there are only 2 types of inserts that actually work and will last - Timesert or Lock N Stitch. Helicoils should NOT be used.

Highly recommended that you use only Motorcraft plugs in the engine. Recommended torque spec is 28 ft/lb, dry, antiseize not recommended or required because Motorcraft plugs have nickel plated threads. The threads in the head do need to be clean and intact.

Have you used a light and a mirror to look down in the well? There could be a bunch of gunk and corrosion down there keeping you from getting the plug out.

 

It could be that in a previous tune up an off brand plug was used that was not a nickle alloy and the plug corroded. In that case when you break it loose like glc says it could take out the threads and it will need either a timesert or Ford uses the lock n stitch repair. The timesert isnt that hard to do but tools are required and if your in a rush the simple solution is taking it in and letting Ford do it.