I ordered two brand new keys from the MB dealer for the 81 300SD and gave them a try today. The keys work just fine in the ignition, glove box lock, and trunk lock, but not the door handles. With the door handles, the new key won't turn unless you pull the key back ever so slightly, and then there's some resistance when pulling the key out. The old key works just fine. The tumblers are original to the door lock as far as I know. My guess is that the new key + the old tumbler = iffy operation. Maybe I need to use a lubricant like graphite powder to make things go smoothly. Thoughts?

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

no graphite; it gunks up. use a PTFE (dry lubricant) after cleaning out the lock cylinders, i like to use the PB blaster creeping oil stuff to clean as it breaks up most of the gunk. you could just spray it in there and let it work for a little or take the whole lock cylinder out and soak it in the PB stuff (the door handles come off rather easy, then just knock out the pin holding in the lock cylinder)

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1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

RIP:
1983 300SD (totalled)
1986 420SEL

Spray some air first into it maybe some dust has accumulated. Safer than spraying a liquid lubricant (which will collect any dust to begin with). Graphite is a dry lubricant so not much of a problem with this.

Also make sure the "wall" of the tumbler exterior (where the key head rests against when inserted) is not caved in. Sometimes through usage this happens. Does the tumbler door work properly?

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My 82 SD is like this. put the key in, pull out slightly, and it works smoothly. my original key is EVER SO SLIGHTLY different from the new one.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Same here. I compared the new and old key. The cuts on the old key and the new key correspond, but the new key's cuts are sharper.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.