Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Or you could make life easy and get a 13mm OD standoff. They will have a partly drilled threaded hole in them that could easily be drilled through to 8MM. Many electronics places sell them, but here is a company that makes them as an illustration of what I mean. http://lyntron.com/catalog/LynTron/B12
I recently bought several couplers from Skycraft surplus, which is a great store here in Orlando, that were multiple sizes. However, I had no difficulty in drilling out one end or the other, or even both ends, to allow it to fit as needed.
Give them a shot, good selection, cheap and been around for a long time. https://www.skycraftsurplus.com/
You may think that is a good idea but most motors have too much speed and not enough torque to directly drive a wheel that moves a car, a cart or whatever. Make sure your motor has enough torque for the job before you incest in the mechanical design effort.