Compiling some manifold data and looking for the runner diameter of a stock twin hs4 manifold, AEG573. Both where the carb mounts and where it mounts to the head (minus the ring).
Have one in the basement. Let me pull it out and measure it.
Inlet side, 1 15/32. Outlet side, ring groove 1 11/32, then 1 1/4.
Basically the same as an hs2, but with a mouth to match a 1.5 inch hs4. Interesting. Thanks for measuring.