* Spark Plug Gap: This is the distance between the spark plug electrodes. The correct gap is crucial for proper ignition. You'll need to consult your owner's manual or a reliable repair manual for the exact specification for your specific engine. This is usually between 0.035" and 0.045", but it's vital to check the correct value for your car.
* Point Gap (if equipped): Older vehicles (though less likely on an '87) might have points instead of an electronic ignition system. A point gap would also need to be precisely set. Again, your owner's manual is your best resource. Since this is a 1987 model, it's highly unlikely to use points.
* Valve Lash (clearance): This refers to the space between the valve stem and the rocker arm. Incorrect valve lash can lead to poor engine performance. You would need a repair manual to find the correct specifications for your engine. Adjusting valve lash is a more advanced procedure.
Without knowing precisely which "gap" you're asking about, it's impossible to give you a definitive answer. Always consult your owner's manual or a repair manual specific to your 1987 Mercury Marquis 5.0L V8 engine for the correct specifications.
