By December 1909 Anzani had a 3-cylinder air-cooled true radial engine running, developed from the earlier 3-cylinder fan configuration engines (semi-radials) that had powered Bleriot across the Channel. By about March 1910 he had completed the first two-row radial engine, a 6-cylinder unit made by merging two 3-cylinder units together, one slightly behind the other and at an angle of 60°. The engine therefore had a lot in common with the early 3-cylinder motors: cylinders were a single iron casting with built-in valve cells and ribs, and pistons were steel with cast-iron rings. The early versions were side-valve engines with automatic (atmospheric pressure opened) inlet valves and exhaust valves mechanically operated via cams in the crankcase. By the end of 1912, as with the smaller engines the exhaust valves were moved to the cylinder heads and operated by push-rods and rockers. The exhaust valves were at the front of the engine, with the fuel inlet manifold at the rear. A prominent pair of 150° exhaust tubes were fitted. Plugs were mounted in the cylinder sides, in the plane of the engine and on the upper side to minimise plug fouling by lubricant.
The crankcase was an aluminium casting and in later models contained a fuel mixing chamber. These later versions also used long bolts to attach the cylinders to the crankcase. The engine used a single, kinked crankweb, coupled to slim but broad connecting rods in order to minimise the axial displacement between the two rows. This was less than in more recent double row radials, the rear face of the front row falling on the centre plane of the back row. The engine was not strictly a radial, as the crankweb geometry slightly offset the centres of the two rows, an arrangement best seen from the back of the engine, where the superimposed inlet tubes and push-rods of the later engine highlight the cylinder centre lines. Such an offset between cylinder centre line and crankshaft is often termed désaxé.
The earliest of the 6-cylinder radials had with a bore of 90 mm and stoke of 120 mm giving a displacement of 4.58 litres (280 cu in) and an output of 34 kW (45 hp) at 1,300 rpm. Its weight was 70 kg (154 lbs). A later version produced 45 kW (60 hp) from 6.33 litres (380 cu in).