Originally Posted by scirocco

Well, I did exactly that today (I really need to get out more )

The ring mounting lugs on a Tiagra FC-4550 are 3.9mm thick, while the 10-speed spacing is only 3.5mm, so the 9-speed rings have a slightly larger gap between them. Nearly half a millimetre difference.

However, since Shimano sell the Tiagra as 9/10-speed, they must have got the Tiagra cranks to work okay with 10-speed chains. Perhaps the lugs on my set of Tiagra cranks are just out of tolerance. I may file them down a little to see if that improves matters.