[alex.s]

Originally Posted by jkv45

If the old plug was not completely seated for a while you may carbon built-up on the threads below where the plug stopped.

I would take the new plug out and spray some carb cleaner on it and on the threads while you try to turn it down with your fingers. You could remove the other plug and put a mark on it (or the socket) so when you turn it in by hand so you know how many turns to expect before it is seated.

I've used carb cleaner before on plugs that had been in the engine for many thousands of miles and didn't want to come out. In that case you move it a little (until it stops) then spray carb cleaner on it, then thread it back in, then back out, then spray, repeatedly until it eventually comes out. Without doing that it would lock-up tightly after 1/16 turn, and most likely mess-up the treads if you used enough torque to continue to move it.

If that doesn't do it, look into a "thread chaser".

I wouldn't leave it how it is.

if you spray carb cleaner in your engine, you must change the oil after.