My understanding is that in 1986, they changed to a 6 cylinder that had an iron block and aluminum heads. The dissimilar metals distorted differently over time, causing the piston to smack the head and bend the rod. In 1985 and older 300's (which was the 5 cyl), the block and heads are both iron.

If I'm wrong on this one, let me know. But I'm pretty sure that's the gist of it.

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1985 300 SD "Col. Klank" - Mileage Unknown
2005 Ford Freestyle - 90,000 miles