How do I measure the quill stem size my bike needs?

Measure the internal diameter of your fork steerer tube with a calliper. Most vintage road bikes take a 22.2 mm quill; a 22.0 mm reading signals a 22.2 mm stem.

What is the difference between a quill stem and a modern threadless stem?

A quill stem inserts into a threaded fork steerer and expands via a wedge. A threadless stem clamps around the outside of a threadless steerer tube.

Can I use a modern 31.8 mm handlebar with a vintage quill stem?

Most vintage quill stems clamp 25.4 mm or 26.0 mm bars. You would need a shim or a replacement stem, which would alter the period appearance.

Which brands of vintage stems are most desirable?

Prominent makers include 3TTT, ITM, Cinelli, Philippe, and Atax. They furnished original equipment on many classic European road and touring bicycles.

How do I know the clamp diameter for my handlebar?

Measure the centre section of the handlebar where it mounts. Common sizes are 25.4 mm, 26.0 mm, and occasionally 25.8 mm. A calliper gives the definitive value.