I have an older OS .61SF and a 12-6 is not enough for it. I found an APC 12-7 works best. A 13-6 also works well. With a draggy scale plane a small diameter prop is not the way to go.

Many years ago we all ran 11-7 & 11-7 1/2 props on .60s. Today's engines have a lot more power so you do get a lot of rpm, but it may not be translating into useful power unless you have a light, streamlined plane, which a scale Decathlon is not. I see lots of people running 11-7s and 12-6s on .46s and for sure on the .52/.53 engines. i still like an 11-6 on a .46, but the planes do fly well with the larger props.