Dan, I've been very happy with the thunder tiger engines. Never had a problem with them and they turn what sounds like pretty high RPM in the air. Never had a tach on them. I always avoided OS because of the liner plating issues they had a while ago. The price was also a factor. There must be something to them. The Quicky 500 guys in New England use the box stock TT40 exclusively.
I spent some time in your area back in the early 60's. I was stationed at Selfridge in an Air Refueling Squadron. My wife came from western Michigan. No ties there anymore.