Truck Repair: glow plugs, ford f350, hex head

Question
The hex head is not there.  All I see the bottom piece of the glow plug.  What do you think would happen if the engine was started?  I have been told that they can sometimes be hard to get out.  Do they make a puller.  
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Followup To

Question -
Hello, I was given a 1985 ford f350 with a 6.9 diesel.  It has a broken glowplug.  I can still see a part of the glow plug.  What do you recommend

Answer -
if the hex head is still there you should be able to take a socket and remove it and then reinstall the new one. thanks for the question.

Answer
you can still start the engine with one glow plug out..they are only used for start up and after start up they are off one wont keep it from starting and that cyd. will fire after warm up about 5 sec. not sure on the puller i would have to check on that.thanks