Old plastic jockey-wheels had only friction-bearings. They were seized and the teeth worn down from the chain rubbing over them. They were also making a massive racket.
New alloy jockey-wheels have sealed-bearings for smooth and quiet running. They come with plastic centre-shims of various diameter. These are the largest bore and still needed drilling out a bit with a 6.5mm bit to accept the vintage screw, which appears to be 1/4-inch rather than metric even though it is French made?
Lower cog fitted OK with the above bore mod and a washer to provide the correct width. The other cog, where the original plastic one had a much larger centre-bore to fit over the metal stud, needed more work.
The stud was cut off and filed flat with a bored-out M8 washer pressed in around it. The rigidity of the cog is provided by the metal bearing itself and is pretty solid.
