Hello, new to the forum, I have a 2000 F250 V10 6.8, replaced the engine at 400,000 miles last October, and now at 30,000 on this engine there's a #7 cylinder misfire, I replaced the COP, injector, and spark plug, still misfire, spark plug black velvet, using a bore scope camera I got this pictures from the cylinder. I'm thinking the rings are done.

 

Don't assume anything without a compression test. Even a burned valve can cause a misfire.

 

What do the other cyl bores look like?

Where did the replacement engine come from?

Before offering any thing the answers to the above 2 questions could have a lot to do with what you might consider doing next.

 

If this problem happened very quickly, I doubt it's a ring issue. That would have been a very gradual process as the compression decreased over time.

 

When we look at the ways this can happen, things like if the former engine owner ran the vehicle without an air filter, sloppy work changing spark plugs by not blowing out the sand and accumulated grit, it just went into the combustion chamber and did a job.

You have your work cut out for you and a little more investigating is on the menu.