I used the largest drill bit that would fit without carving up anything it ain't supposed to (19/64s) by hand on the ones I could get it in. But as you work your way back there is less room and on #5 no room for a 4-5" long drill bit. I considered cutting a bit shorter but immediately realized that I would end up dropping it in, and was not going to run that risk! So I dug around in my junk metal pail found a rod that was over 1/4 " and under 5/16ths, cut it to about 5", cut some little flutes with the dremil, heated it, bent it in the middle so it was l shaped. It isn't pretty, but it worked perfectly, and because of the bent part I was able to rotate it and still not drop it in.
Someone else here on another thread took an old GPlug and just hammered the end flat so it was a bit wider/longer and threaded it in and out. If I had had a bad GPlug handy I would have done that.

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Thanx,
Alberta Luthier
1983 300CD ('Stinky')155k miles, 2.47 diff, EGR removed, AAZ injectors with 265 nozzles from Sean,and vogtland lowering springs.
1984 300SD ('Old Blue')150k Klicks from Japan originally, came with rear head rests, no sunroof and never had an EGR