This came up a while back. There was a question about the ID of the "small" venturi on the OS-20 and 25-s baffle piston engine from the 70s.  While I am away from my resources, I did finally find one in an old hobby shop (after looking for about 35 years...) and was able to measure it. The ID is 5.5 mm or about .217". With a spraybar diameter of ~0.123", this gives a choke area of about 0.0118 square inches. It came with a "small" and "large" venturi, but my experience was that the "large" was pretty hopeless on suction.

    This is well under the "magic area" from the other thread - which means you can't expect it to pull airplanes of the typical size range. And it won't.

    It's a nice runner, or at least it was in 1979 when I last flew it, before breaking the crank (and not finding a new one until the 90's) and giving away the original venturi for another engine (and not getting one until this weekend). But it will not make it on something like a Skyray 35. You need something in the range of 300-350 square inches, or have overkill on a Shark 15, Jr Nobler, et. The last airplane I designed for it was around 350 with flaps and a pretty thick airfoil.

  Anyway, I forget who was interested in this, but that's what it came with.

    Brett