Thanks for the help. I bought a $2 M6x1.0 tap; it only went in a couple turns until I felt moderate resistance. Looking in with a light and a mirror I could see the first 4 threads or so were nearly gone. I tried my M6x1.0 bolt and it went in with about the same resistance as the tap, so I just screwed it all the way in using a nut driver. I bought a longer bolt and had it threaded at a machine shop for $2 (I offered him $10). It worked perfectly, using an old valve guide. Total cost: just under $6 and far too many hours. Had I known the correct bolt size it would have taken an hour. btw, I had forgotten to order the tool along with my other parts, but yesterday Mercedesshop gave me the best estimate of $17, including shipping vs various MB dealers at $45 to $55.