Different Spark Plug Types in Same Engine

[KMurphy]

What is the reason for Ford to put 3 copper and 3 platinum electrode spark plugs on some V6 engines? One of my co-workers has this arrangement on his late 90's Explorer and the manual specifies this for replacement to. I googled it, but couldn't find any relevant information. Thanks to any who possess and share this information.

 

[Eli]

WTF?

That makes absolutely no sense to me.. and I have no idea.

 

[Howard]

The assembly line workers ran out of stock on one kind.

 

[Vic]

Ford occassionally used different spark plug types for the harder-to-reach cylinders, usually so that you didn't have to replace that plug as often. IIRC, some V6 Aerostars actually had a permanent spark plug on one cylinder because it was impossible to R&R that spark plug while the engine was still in the car.
Can't tell you if that is the case here with your friend's car.... but otherwise, it doesn't make any sense.

 

[KenGr]

If it's copper and platinum and the plugs are otherwise identical in heat range, it was just because of the difficulty in replacement (and the fact that platinum plugs used to be horrendously expensive). If I had it, I would just replace the three copper plugs with platinum and then not worry until it hit 100K miles.

Back in history, a few engines used different plugs due to cooling problems. Probably the most famous were the Arial Square Four motorcycles which could not effectively cool the rear cylinders. Some owners of aircooled longitudinal V-Twins (like Harleys) sometimes run different plugs in the back cylinder due to overheating problems.