I went out this morning and the beast would not start. Running late I did notice the glow plug light was not on. Interior lights would dim, but I am betting another glow plug has bought it.
Question is, my recall is the small glow plugs (81SD) use a 12mm wrench or socket wrench, and does Sears carry a 12mm 1/4 drive deep swivel? (assumming that is the correct size.)
I would like to get one of these asap.

I know the loop style glow plugs takes a 21mm wrench. I bought a set of ratcheting box end wrenches for this...

That is what my 78 has, the 81 has much smaller glow plugs.

but I a guessing you can still use a 12mm ratcheting box end wrench on the pin style glow plugs ...

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)
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Found three glow plugs that were not functioning. Short life. The two rear ones were out of course. Those are the hardest to access.
Not so bad getting them out this time. When I last replaced them I used a little anti seize compound on the threads. Came out easily.

If you got really short life span out of them... you may need to check your injector spray pattern....
also... I think I saw somewhere on some of these engines... if the vacuum line lets oil be pulled back towards the top end it can cause things like precombustion chamber cracking... have no idea if that in any way could apply to your engine... but I read it in the FSM and wanted to tell SOMEONE about it... LOLOLOL

Leather,
Interesting thoughts. I have a head from one of my parts engines in the shop now for a rebuild. Then it goes on the 81. They are going to pull all of the prechambers and check them over before the hot tank and the valve grind.
Maybe with a new head I will get some more life from these glow plugs. Not so good right now.