I ground a clearance cut in the rotisserie mount with a large sanding drum so I could drill the hole for the Alternate Air control from the engine side of the firewall. Drilled a pilot hole with my smallest step drill. Used a larger step drill for the final hole. Cleaned up and deburred the hole with a debur tool and finally a Dremel tool with a sanding drum. The hole turned out to be very round and a 0.618 OD socket with one layer of tape (0.626") fit through nicely with a very small and even clearance all around. A dial caliper showed a hole ID of 0.0640" at one location. My goal was 0.625" - 0 + 0.030 and I think I achieved it. The hole for the Mixture control was straightforward with great access on the engine side and good access on the cockpit side. The Hercules cobalt step drills that I purchased this morning didn't work as well as the Warrior titanium step drills I had already been using for smaller firewall holes. So I made a Harbor Freight run to get the larger Warrior titanium bits. I finished the Mixture hole with the Warrior step drill, but I'm not sure it was a lot better. I think the larger size jump between steps and the larger hole diameter made it not work as well as the small Warrior step drills. I had to work on the larger holes with a debur tool and a Dremel sanding drum to clean them up. The resulting hole turned out nice. I measured the hole ID of 1.140 after using the debur tool and 1.150 to 1.162 after using the Dremel sanding drum. Image 7907 shows a 1.158" socket will not fit through the hole. A 1.129" OD socket (socket with two wraps of tape) fit through the hole easily with a small even gap. My goal was 1.125" -0 + 0.030" and it seems I missed it by 0.005 to 0.010". The hole for the Throttle control was more difficult with only access at an angle on the engine side and okay access on the cockpit side. I used the small and then large Warrior step drill. The resulting hole wasn't as clean as the hole for the mixture control and required more work to debur and clean up. The final hole was not as round as the mixture hole. Image 7917 shows a 1.158" socket will not fit through the hole, but there are approximately 0.030" gaps in some areas. A 1.129" OD socket (socket with two wraps of tape) fit through the hole easily, but there was a larger, uneven gap compared to the hole for the mixture control. My goal was 1.125" -0 + 0.030" and it seems I missed it by 0.030 to 0.035". I'm still confident it will work.