Once you start forging that jack hammer bit, it's going to fit quite snugly in the hardie hole. You will be forging square, so if you hit it on the corners first, it's going to upset larger than 1 inch, then just taper until it fits the hardie. I do endorse the Brian Brazeal type rounded hardie. You have to treat your tools like they deserve to be treated. The rounded shape allows you to easily start with the heated steal on the far side of the semi-circle and to roll the material toward you as you cut, then you can push it away from you with your hammer head and cut as you roll back toward you. This allows you to avoid overheating any one section of your hot cut. It also provides for a more efficient cut, because the rounded shape of the blade reduces surface area contact. No need to bulldoze your way through the material.