I just discovered that the two Supertigre BG60 Blueheads I have use different size throat diameters. The engines are identical other than that. I haven't measured them but the handle of my #11 x-acto knife will not go in the small one and goes in the larger throat very room to spare. I just measured the handle and it appears to be 8 mm. So one throat diameter is less than 8 mm and the other is more, maybe 9 mm. I flew both of these engines for years in the 70's and did not realize the difference in throat size or in power output. I flew them in my 78 Nats winning Fliteglas P-51 and another Flitglas P-51 that I flew over 1000 flights. Still have both P-51's by the way.
Ralph White, Neoga, IL