Notice the pair of grooves in the rollers? One pair of grooves is wider than the other. If you're changing wire diameter, you'll also use a matching roller. The rollers have markings indicating the wire sizes they fit. Ours fit 0.030- and 0.023-inch wire. Install the desired size with the markings facing up. In other words, if we were to install thinner 0.023-inch wire, we'd flip both upper and lower rollers over.