Hi all my brother has put an old quill stem in his cheap modern commuter and although it's tight (stem turns when fork turns) it wobbles insitu as the diameter is smaller than the opening at top of head set.

He's been riding it like this for ages but I'd like to fix it for him.

Thought about a sleeve but it would have to be deep so it meets the expander if this would work.

Ideally though a stem of the correct diameter would be fitted.

What is the diameter of a modern stem (if there is a standard or are they proprietary.

I'd measure it but he lives across town so want to do some research here about best practice.

Thanks in advance, Matthew