Don't worry about the collar, it comes out with the tumbler. Also the right size wire diameter is not the size that will fit in the outer holes. it necks down somewhat inside when it gets into the grooves of the tumbler.

The collar is wider down below; the tumbler fits inside the lock body then the collar covers both. it is free to spin. With proper tool this is less than 5 min job, including drinking your coffee. unsure of which position the lock has to be in, mine works one detent ccw from the run position. Its the one where the radio wil still play. Good call to fire the "locksmith". Most of the chassis shop manuals will have a sketch of the factory tool, with dimensions.

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Hanno
'79 6.9 Sold (after 27 years)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon