In my opinion this is the best way to fix the odometer and trip meter in the Mercedes W123. First I'll explain how it works, and then get into the repair. Scratching the shaft alone is a decent fix but without the loctite it's not robust for life. This video might work for a W124 or W201 also. This fix is still holding strong in my W123 and I expect it to last the life of the car.

Materials:
Loctite (DONT use the red stuffget the blue)
10mm wrench to remove the oil sender line from the cluster
Basic hand tools like assorted size flathead screwdrivers

Song:
SLPLSS Drive