If your key was hard to turn, the tumbler could be toast. A new key wont help this...it may make it worse since the new key will be a tighter fit.

Just a thought - you could get a whole new tumbler assembly from MB, with matching keys to your VIN. I bet you could order one through Duval Motorcars in Florida if Phil here at mshop cant get one for you.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car