Do not pry and risk dropping the tip of the glow plug into the combustion chamber. Simply attempt to remove the swollen glow plug. If it becomes risky you can remove the fuel delivery nozzle from the effected cylinder and use a curved jaw vise grip or a set of locking hemostats and attach that end to the swollen glow pug tip. Now have at it. Pry or slowly break off the glow plug. Use care. I have done hundreds this way. I have also removed inner fenders and steering column lower shafts to get in there and get the job done. Whatever it took to get them out. I bought a few from DRMO that had been code H for swollen plugs, and a few had the plugs hanging out the holes and required surgery to get them out. On one occasion I had a glow plug fail while driving and knock around in the cylinder a few revolutions and then just have the sound fade away. At the time I did not know what was happening and at 60 MPH in an M1028 that gets your attention. I continued on and went several hundred miles that day. The next morning, I experienced hard starting and pulled the glow plugs and found one with the threaded part only. That solved the strange noise mystery. Never hurt anything that I was aware of. probably was inside the muffler at that time. Good Luck. Be Careful and do you lube on tight/swollen glow plugs. Steady even pressure and extreme caution. Removing the nozzle is an easy task. Have a Great Day.