.864" is what mine measures. I was in the process of building one too, but only had .750" stock available, so that is what I used to start my build. Didn't want it oversize and break the shaft mount bosses. I didn't finish my tool. I was hoping to not have to remove the heads,,but after the motor got here and a leak-down test,,I have a cylinder with excessive ring leakage, SO, I will use my off head tool.