That’s a good question Chaplin37,
In your previous post about rusty hands you were referring to an American pocket watch so I’ll address American pocket watch mainsprings here. It’s always a good idea to grab any movement reference books that you can find. The “Illustrated Manual of American Watch Movements” is a great one to have in your library. If your movement still has the old mainspring in it you can take some measurements and place your order with those dimensions. The supplier will also need to know what type of end is on the mainspring. You can easily determine the correct type and name by comparing it to one of the many diagrams towards the back of the book. You would then need to measure the Width (height), Thickness (strength) and Length of the spring. There are two ways to do this. One is with metric and inch the other is Dennison. There are Dennison gauges available from material houses. If you are going the metric route, using a micrometer measure the width and thickness in mm. Then measure the length in inches. This information along with the type of end will be all that the supplier requires. The problem with this method though is that we are assuming any prior mainspring replacements were the same size as the original. It’s not too uncommon to find watches with the wrong size mainsprings in them. A watchmaker may have replaced a spring with the closest one he had in stock or may have used a stronger one to over come (power through) a problem the watch is having. Using too strong of a mainspring will cause undue wear on teeth, pivots and unjeweled holes. A proper repair of course is always to fix the problem and stick with the correct spring. For this reason a better ordering method is to use the same book and try to identify the movement model and grade. The mainspring number will be listed under the grade. All that is necessary then is to pass the mainspring number along to the supplier.
If you’re going to be working on American watches then I highly recommend that book. It’s by The E.&J. Swigart Co. and I believe it’s still available through some suppliers.
Hope this helps Chaplin37,