The title is pretty self-explanatory. I realize they have different part numbers, but does anyone know if they're dimensionally different?

 

No, 991 spindle is larger

I have a set of 997 grey ones that are in perfect condition if interested.

 

Some people have reported doing it - are they changing just the cover on the nut? Is that the same size?
I want to change my 991 from black to silver, but they don't make the 991 part in silver. Can I put the front component of the 997 on it?