Originally Posted by Wilfred Laurier

Be careful when ordering a 'one inch' quill stem - the quill diameter on a '1 1/8"' stem is 25.4mm (exactly one inch). The diameter of the quill in most 1" assemblies is 22.2mm (approximately 7/8"). Many 'one inch' assemblies also used 21.1 mm diameter quills, although this was usually only common on very low cost bikes.

But if you order a 'one inch quill stem' and your current stem is 25.4mm in diameter, the new stem you receive will not fit.