Your SD has the same 617 engine that is in my '83 TD. the glowplugs are 12mm. I use a ratcheting "dogbone". There are a couple of plugs, I think #2 and 3 that you can get with a socket. However, for the rest there isn't enough room for a socket and ratchet. I have heard of people removing the fuel lines to get easier access. Personally, I would rather not disturb those lines.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member