1999 Cobra knocking after spark plug blow out

Hi,
Sorry to hear, all too common on these, unfortunately, not nearly enough threads for the material type. They were “Close”...only effective things that are useful close are when throwing Horseshoes or hand grenades..
So, you HeliCoiled the plug hole with the head installed on the motor? How did you address the chips in the Cylinder? Would you classify more as a knock, not a Tap, possibly a “hiss” from escaping gases, and more top or bottom end?
When the sound is heard, is it consistent with Crankshaft speed, or 1/2 that- Valvetrain speed?
Good way to address this is using a Time-Sert, better yet with a full girth thread insert. Helicoils work, but rapid thermal expansion of Aluminum leaves little thread engagement once hot.
http://www.timesert.com/
Best!
-John

vb7200 said:

It shows up at low idle, it’s almost non existent from 800-1800 RPM. Once above that you can hear it. It sounds like it’s coming from the bottom center of the engine compartment.
To get the shavings out, after tapping I hit the cylinder with compressed air, and used a core scope to check for chips, all looked well before I put everything back together to start it up. Putting my ear right up to the plug that blew, I do not hear any sort of “putting” like gas escaping. I heard a similar sound every once is a while when backing up, if I pushed the clutch in fast it would make almost the exact same sound until I came to a stop.

Click to expand...

Hi,
Only objective, here...Worst case...Compressed air? Chips likely went everywhere, ending up between Piston & Cyl wall sitting on top of the compression ring, which may stick if chips end up behind the ring, galling the cylinder. hopefully none pinched on a valve seat.
Job’s done, nothing can reverse any damage done, if any, at this point.
What to do about it?
Change your oil, your filter more frequently a few times. Cut the filters open & look for chips, drain the oil & look at the oil pickup with your Borescope through the drain plug for shavings.
Regarding this repair impacting things there’s really not much you can do but monitor codes, and the health of your engine at this point, compression, leak down, oil pressure monitoring, and any new noises.
IMO You wouldn’t note lower end noises from negative effects of chip contamination from this very recent repair quite this early.
I’d be happy to help you troubleshoot any noises you may have, what you’d listed sounds like a clutch related issue, possibly one of the below may help..
FYI’s on your Cobra...
The 99-01 Tumble-port heads, moreso the 99’-00’had a cooling deficiency around Cyl’s 6-7-8 (Drivers side) causing the valves to get hot fast & were known for making a random “tick” or “ping” noise, similar to a subtle spark knock. Ford TSB on this.
The Bosch traction control systems on these Cobra’s pull timing under hard acceleration, when older- May add/pull whether disabled or enabled, if you’re hearing something, a ping, turn TC on or off, or opposite of how it’s normally run.
I suggest swapping oil & keeping tabs on this, try to pinpoint the noise closer with simple detection tools.Any other questions or comments, please feel free to post.
Many viable options out there the repair things like this with all the Modular motors creating demand. I do suggest considering removal of the head during the off season & performing a solid repair on any questionable plug threads.
Good luck!
-John