I’ve always liked Elmer’s No.5 geared engine because it’s such a weird design but the ring gear is just too expensive for me to justify to myself. Playing around with the geometry it looked like it should be doable with almost any ring gear so long as you adjust the gear arm length to keep it on the center line and have a 2 to 1 ratio with the pinion gear. https://www.homemodelenginemachinist.com/threads/elmer’s-5-geared-engine-maybe.30206/ For this engine I used a gear out of a dead Ryobi cordless drill and made the pinion gear. Most of the build is straight off the original drawings with only the drive end reworked to work with the gears I had. I’m afraid I went for looks over function with the flywheel and the engine would probably be able to run a little slower with something heavier. So if anyone has wanted to try this engine but was being held back by the cost of the gear just get a dead drill and go for it. In the worst case you can reuse most of the parts and make any of Elmer Verburg other engines that use the “M” type cylinder.