We discovered this engine brand in the American “Aeronautics" magazine and that made two models of air-cooled engines with upright, in-line cylinders.

-They were the 4-cylinder that gave 30-40 HP and the 6-cylinder that gave 50 HP. Both were called “Gray Eagle”.

-This name was also used by the Kemp engines. And even the engines were remarkably similar. But there is no other parallelism that allows the author to find a common thread or source.

-The Kemps were manufactured by Kemp Machine Works in Muncie, IN. and the Rubels by R.O. Rubel Jr. & Co, Inc., in Louisville, KY.

-But I insist, the architecture was extraordinarily similar in both brands.

"1911 Rubel Gray Eagle engine"

-Between this 30-40 HP engine and the next one, which is advertised as the same model, the difference is that it is from 1912 and has a rectangular oil pan.

“1912 Rubel Gray Eagle engine”

-It stands out that the intake and exhaust valves are concentric so that they are in the same body. As in other engines of the time, the difference is in the route to match the ports with the gas passages.