I believe there is a special tool designed to lock on to the flywheel teeth to stop the crank from turning. Without the special tool, your best bet is probably a large screwdriver or small pry bar wedged into a flywheel tooth.

Do I recall correctly that there's some kind of rubber piece bonded to the harmonic balancer that can fail, causing this kind of problem. If so, it could be that and not a key/keyway problem.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13