tankatoy

Hello everyone I recently did a tune up on my Jk with only one prob the middle spark plug won't go back in all the way it only goes In half. When I first took out the spark plug i t gave me some trouble. The plugs haven't been changed since god knows when since it was a used truck. Any help would be much appreciated . Also alil worried that it might have to be treaded . If so would it be expensive if it has to be done