glow plug starts to unscrew when trying to replace then tightens
 

when trying to replace the glow plugs they have all been easily replaceD excepT the middle one which loosens then about half way out it tightens again so I HAVE STOPED IN CASE IT BREAKS IN THE HEAD. ANY TIPS AS TO WHAT i SHOULD DO?

tKS AMAC

keep screwing in and out, slowly , spray some rust remover and lubricant in there and go very easy, go all the in and then slowly back it out to were its tight again then go back in and then back out again, easily and carefully. dont force anything, otherwise your F'd , or just leave that one in..if worse comes to worse.