Per dimensions of the glow plug in this ebay listing New Genuine * TRIDON * Glow Plug TGP For Mercedes Benz E300 D E300 TD W124 W210

28 mm is the portion of the tip that extends into the pre-combustion chamber, from the flare to the glow plug hex (assumed where it snapped off) is about 2 x 28 mm. 56 mm is 2.2 inches. Did you drill more than 2 inches deep?

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