I'm not yet convinced that you have any foreign object in there. A small, flat washer, maybe, but I'm still not convinced because anything in there should be accompanied with a lot of clicking or banging or other awful, mechanical sounds.
I had this happen to a friend sometime back in the 1960's (yeah, I'm that old...get over it) and this is what we did:
1.) remove the offending plug
2.) get a 12" welding rod or piece of coat hanger - make sure it's straight.
3.) get a 30mm box wrench (please don't use pliers or a crescent wrench - it just screws up the crankshaft pulley) with which to turn the engine over by turning the crankshaft at the pulley.
4.) gently put the welding rod down into the plug hole until it bottoms out. Keep gently holding onto it while yourself or a friend slowly turns the engine over in a clockwise direction (FOR HEAVEN'S SAKE, MAKE SURE THE IGNITION IS OFF!)
5.) you'll feel the wire going up or down. Keep turning it over slowly until the wire hits the top of the stroke and starts back down, then move the wrench slowly back and forth to find the top of the stroke where the wire is as high as it can get.
6.) reach in and make a "sharpie" mark on the welding rod exactly where the outside of the plug hole is on the welding rod.
7.). Remove the welding rod and write down the dimension from the end of the rod to the bottom of that mark - do this carefully for accuracy. This will eventually give you your head to piston clearance (Patience!)
8.) Turn the welding rod around and bend the last 1/4" at a 90° angle. Not 85.5°, not 92.7°, but a straight 90°
9.) Turn the engine over 1/4 turn or so and insert the bent end of the welding rod into the hole far enough to turn it a bit and catch the inside of the head. Hold onto the rod, reach over and put a "sharpie" mark on the rod where the outside of the head touches it.
10.) Remove the rod and measure from the inside of the bend to the inside of the mark you just made and write it down - that is the thickness of the head which should, BTW, be about the length of your plug threads, but we'll get to that.
Now for the good part: Subtract that last dimension from the first one you took to see the difference. That is the space between the top of the piston and the inside bottom of your head.
10a.) Now, get your suspect plug and re-insert it as you had it originally, torquing it to the proper spec. Pick a random spot on the plug (I always use the top of the socket hex because it is easy to see/get to) and measure from the outside head surface (where the plug goes in) to that top of the hex and write that down as a reference. If it's difficult to get a small scale down in there, just use the un-bent end of the welding rod and mark it (Use a different color sharpie).
11.) Remove the plug and measure from that top of the hex (which you just referenced - use the same spot) to the bottom of the plug electrode and write that down. Subtract the shorter length from the longer - that is the depth of the plug that protrudes into the chamber.
If the protrusion length is the same as the chamber height clearance, you have a mechanical interference (the plug protrudes too far into the chamber. There are two things I would consider: a.) Make sure the plug has the proper crush washer on it - not a re-used one, but a new one, crushed to the proper torque or;
b.) get a shorter reach plug. To consider this, I would look at the available chamber space, how thick the head is and then find a plug with the proper thread length to position the electrode ground tip in the middle or top half of the space. I would have enough clearance to never hit. I would then swap out the other plugs with the same type, too.
Only if I found adequate clearance to not interfere would I then look for something inside the cylinder. You could, of course, check your other cylinders this same way, just to see if they are all the same.
