EASYTIGER said:
"Small head WILL NOT work with byron."

I usually stay out of these discussions, but sometimes I just can't help myself when someone is so emphatic about what will and will not work when my real world experiences are the opposite. I flew three Byron F-16s for six years. I flew a Regal Eagle for 1 1/2 years. I had two Top Gun F-15s, and one Byron F-15 twin. All of these had Byron fans. With the Dynamax fan I had two Yellow F-16s, two Yellow F-18 twins, and one BVM Aggressor with an OS .91 in a Viofan. At one time I owned 12 OS .91 engines. Only two of them had the large head. My flying buddy Vernon had more Byron planes than I had with a comparable number of OS engines, and his experiences are similar to mine. In my experience, the small head OS .91 engines will run fine in a Byron fan with the small head. Whether you use them in a Byron fan or a Dynamax fan, use red loctite on all the case screws before running. This will greatly improve reliability. Also contact Tom Cook for a heavy duty throttle arm. It is only a matter of time until the stock one breaks. Peak the engine out, then back off until you get a good smoke trail and let her rip. Wildcat 5% fuel with 20% oil will give great top end with a really good idle and transition. Also, I have never slung a rod in an OS.91. Just don't run it lean and you won't have a problem.

Good luck,

Dennis Lott