There are two options to measure the mirror stem diameter. You can measure it using Calipers or if you do not have calipers you can use an adjustable wrench and a ruler. Please note that you have to have a round uniform mirror stem with at 1 inch of the stem visible to be able to use the mirror mount.
Calipers: If you have access to calipers use them to measure the diameter of the mirror stem and record the measurement. (We prefer the measurement in mm if possible but please let us know the measurement unit either way)
Adjustable Wrench (Step 1): Close an adjustable wrench on the mirror stem and carefully slide it out.
Adjustable Wrench (Step 2): Use a ruler to measure the opening in the wrench. (We prefer the measurement in mm if possible but please let us know the measurement unit either way)