Piaggio acquired a license from Gnome et Rhône in 1925 for their engines derived from the Bristol Jupiter.[1] The designs proved successful and, using this experience, the company designed a range of related radial engines named "Stella", meaning star.[2] The design was led by the engineer Renzo Spolti.[3] The engines were initially known by their number of cylinders, so the first nine-cylinder model in the range was the P.IX of 1933. However, a progressive number in Roman numerals was used as the design progressed, so the P.IX was followed by the P.X.[2]
The Stella P.X was a nine cylinder version of the P.VII.[4] It retained the same bore and stroke as the original Gnome-Rhône designs, 146 mm (5.7 in) and 165 mm (6.5 in) respectively, but was substantially more powerful than the comparable 9K.[3] The engine had a two piece aluminium alloy crankcase, steel barrels for the cylinders and aluminium alloy heads. A Piaggio T2-80 updraught carburettor was fitted.[5] The basic version, the R., had reduction gear, while the R.C. was also equipped with a compressor.[2]