13RGU fork inside diameter?

Hi everyone, I am going to make air pistons for my front end. Does anyone know the ID of the front fork tube on a 13 RGU with 41mm forks. I know I can buy them, and I've already talked to Big Al at Dirtyworks, but I have a lathe and want to give it a try. I read an article of a guy that made some for his Fatboy. It can't be that hard, it just a cylinder with orings. If it doesn't work out, I'll call Big Al for an order. I trying not to tear it down just to measure, as I am not quit ready to install, and I'm hoping to make them before I tear down the bike. Thanks, adown