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07:12
So that dull silver tube is a bushing that is supposed to go inside the idler wheel/cog
its supposed to be flat..... its not cos the chain's been driving it round
At the moment the chain is running through that black plastic chain tube straight below the bushing
 
17 hours later…
23:45
I remember that you had trouble with that when you restored the drive train the first time.
There's no idler you can jury rig, perhaps out of a jockey wheel?
If you remove the bushing, cut some spacer out of a tube to centre it.
Or is the scale too far off. Can't quite judge the size from the close up photo.

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