I has Enya 41-4c diesel, the glowplug does not have the filament inside, the glowplug are made as a blindplug.
See the last photo of the cylinder head with glowplug as blindplug here:
http://www.rcuniverse.com/forum/fb.asp?m=5520292 and the other photo here:
http://www.rcuniverse.com/forum/fb.asp?m=5525676 .