Some seemed to think this was a good idea, for reclaiming those glow plugs with the posts broken off from practicing 5' bottoms a little too zealously...it's a split-pin dealy, I think intended as a shear pin, I found at the local hardware store. Pried open with a screwdriver and forced over the post stub (carefully, so you don't break the seal of the stub). The pin may then be cut down with a Dremel wheel or something similar, but I'm too lazy...it works, I leave it alone.
--Ray