Hey Bevo, I.m pretty sure forks are sized by the outside diameter of the inner fork tube. In other words, your fork seals will have an inside diameter of 41mm and therefore will fit inside the outer tube which will have a different and larger inside and outside diameter, like 55mm or something close to that.
I don't think your 41mm spec size of your forks will coincide with how turn signals are sized. My guess would be they are sold to fit the outside diameter of where you want to mount them. In the case of a stratoliner/roadliner, turn signals may be mounted to chrome covers that dress up the outside of the fork tubes. For what you are doing, I would simply measure the diameter of where you want to mount the turn signals and use that measurement to order them.
The easiest and most accurate way to measure the diameter is with a set of calipers. If you don't have those you can wrap a cloth tape ruler around it and take that length (girth or circumference) and divide it by 3.14. If your cloth tape measures in inches you will have to convert it to millimeters if that is how the turn signals are sized.
Last edited by stratowart; 01-29-2012 at 06:01 AM.