A bit bigger than 1/6 smaller than 1/5 would guess about 1/5.5 or so. The dummy radial stinks but its OK for a ARF. If Williams Brothers comes back with a kit then you could make a 7 cyl with a new crankcase. I will do that to a Ziroli Stearman after I wear out my GP Stearman. I just enjoy it for what it is and don't want to spend more time changing all the things that would make it a better looking scale plane. Its easier to start a build from plans and make a nice Stearman IHMO.

I just did about 25 touch and goes yesterday. Its a good flying plane.

Carl