Spark Plugs do not fit

I am having a problem with fitting new spark plugs. It seems that the plugs I just removed have some added collar on them. I removed them only to find that the new plugs are too narrow for the holes. Both the old and the new are Champion N9YC. I feel like I'm losing my mind. I've ordered comparable spark plugs only to have the same issue. I have an old pair which I removed a while ago and even they don't fit in.
I did have the bike serviced a couple of months ago.

Is there a thread insert stuck on the old sparkplug. Helicoil or Timesert? My guess is a thread repair that came out on the plug. Find out what brand it is and how it should be inserted so it stays put.

Both spark plugs seem to have them. If that is the case, where do I go from here? Can I remove them from the old spark plugs without damaging the threads or can I find replacements?

Can you post a picture of the plugs

PatMat said:

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Yup. Like Carl said the left one has an insert stuck on the threads

Those plugs are very sooty. Are you running with the chokes on? And if they are Champions the 9s are a hotter plug than standard N7Y.

I have only used helicoils, But the ones on your plugs doesn't look like one. Try to use a padded jaws on pliers and try to turn if off Gently. Maybe clean all the soot off with brake cleaner and see if it spins off easily..

The inserts could be bonded with "Loctite" so heat might be needed to break the bond.

Take a picture from the other side/top so I / we can see the insert end s. You really need to figure out the brand of the insert to replace them if the old ones get damaged.