Why would anyone want to change it? Well, the one that came with my engine was setup for a Hot Prop or deicing propeller. As you can see in the first picture the hot prop gear assembly has greater mass than the standard ring gear assembly shown in the second picture. In short it's a way to save some weight in the nose of the RV-10. The hot prop assembly weighs 8.86 pounds. Were as the standard ring gear assembly weighs 6.9 pounds.A couple of things to keep in mind about this change out is to use the same tooth count on the starter ring gear that matches the starter being used. They come in 122 and 149 count. Also, you will probably want to stay with the same alternate gear pulley size. The standard Lycoming size is 9 3/4". This is the size recommend for use with Plane Power alternators.
While changes such as this are legal on homebuilt aircraft, it can change a certified engine to an experimental engine depending on the part used. Also, it's important to document changes such as this in you engine log book for later reference.
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