Spanner, your B-17 engine tribulations sound very familiar. . .
A few years ago, Chuckie was on her takeoff roll and the #2 engine started making "less than ideal" power and doing a lot of shaking. The takeoff was aborted and the ship was taxied back to the ramp. Steve and I investigated the problem after letting the engine cool down. The culprit turned out to be a shattered insulator on a spark plug, causing that one cylinder to misfire.
A couple of years prior to that, our #3 engine was smoking a lot and using a lot of oil, and it kept getting worse. After a particularly smoky flight home from the Denton air show, our intrepid mechanics tore into the engine to find the cause. The problem was that an incorrect set of rings had been installed on the #5 piston when the engine was overhauled. The rings caused excessive wear, which led to replacement of the piston and cylinder. That was a real pisser for us because the slug and jug were both brand new when installed!

The piston is sitting on my desk here at work, right next to a piston from B-25 Pacific Prowler (quite a sight for the un-initiated!). I think the cylinder ended up with a collector in Germany.