Taking accurate measurements of your balance staff is crucial in getting the proper replacement.  I utilize a “JKA Feintaster” dial gauge made in Germany (see image below). It is a very accurate (read – expensive - $600+ in 2014) dial gauge.  It measures to 1/100th of a millimeter (or 0.00039 inches). Of course it, like any measuring tool, will only be as accurate as the person using it.  

Feintaster


To compound the difficulty in getting the proper replacement consider the following passage from the The Chicago School of Watch Making Home Study Course Material:
“The dimensions of balance staffs vary for different models even in the same size and make of movement. One of the older American factories, now out of business, had eight models all of the same size, each using a balance staff of different dimensions. This is usually due to a change or refinement in the model.”


Don’t go only by a part number listed in a book or on a website.  Take the time to accurately measure your balance staff.  If you provide the dimensions (in the same format I use) and the part number of the staff you’re searching for I will do my best to supply either an exact match or the closest one I have to it.  

Here is a diagram of the necessary measurements to ensure you get the proper staff:

Balance Staff Dimensions


As with all replacement balance staffs it may be necessary to “tweak” them as necessary in a jeweler’s lathe for a perfect fit.  Also, keep in mind that these staffs (even if they are NOS – New Old Stock) may have been manufactured 100 years ago or more.  They may possibly have some surface corrosion or rust.  I will send you the “best” example I have available when ordered.  I will not send one that I don’t think will clean up to useable condition.  I personally examine each staff before it’s shipped to make sure it is in good condition.