The Alvaston firm in the UK was an early manufacturer of aircraft engines. All of their HOAE were liquid-cooled and had a flywheel on the crankshaft, presumably to reduce vibration and the variations in thrust and torque that resulted from engines operating at low rpm. A contemporary reference is the 1920 book by E. Charles Vivian (ECV) in which a chapter covers horizontally-opposed aircraft engines. The Alvaston engines are described as notable engines among several HOAE from various countries. Vivian's book is available on the Internet and can be found here. Another reference is Setright's book (LJKS). The aircraft applications listed below are from the well-illustrated reference, "British Aircraft before the Great War", by Goodall and Tagg (G&T).
OL-140 -- {4.488 / 4.488 / 142.0} / {114 / 114 / 2327}
Same bore as OL-320.
2cyl; N/A; 20hp@1200rpm; 1909-1914; Wt = 95#, less flywheel.
Liquid-cooled, single-ignition engine.
Ae39; G&T (for applications); J1912.
Applications: (UK) Gaunt biplane No.1 of 1910 (built, but not flown), biplane No.2 Baby of 1911; Gibson biplane No.1 of 1909 (built, but not flown), biplane No.2 of 1910; Gnosspelius hydro-monoplane No.1 of 1910 (built, but not flown); Hartill monoplane of Demoiselle type of 1910 (built, may not have flown); Stewart monoplane of 1911 (built, may not have flown).
OL-210 -- {5.197 / 5.000 / 212.1} / {132 / 127 / 3476}
2cyl; N/A; 30hp@1200rpm; 1910-1914; Wt = 120#, less flywheel.
Liquid-cooled, single-ignition engine.
Ae39; G&T (for applications); J1912 (with photo).
Applications: (UK) Avro Duigan biplane of 1911; Batchelor monoplane of 1910 (built, but not flown); Central Aviation monoplane; Chittenden & Robinson biplane No.2 of 1910 (built, only hops achieved); Collyer-England biplane of 1911; Crompton monoplane of 1911 (built, may not have flown); Dawson & de Pavillet monoplane of 1910 (built, only hops achieved); Handley Page Type C. HP.3 monoplane of 1910 (built, but not flown); Nottingham monoplane of 1911 (built, may not have flown); Yates monoplane of 1911 (built, but not flown).
OL-320 -- {4.488 / 5.039 / 318.9} / {114 / 128 / 5226}
Same bore as OL-140.
4cyl; N/A; 50hp@1200rpm; 1910-1914; Wt = 160#, less flywheel.
Liquid-cooled, single-ignition engine; J1912 states that the stroke is 5.000 (127) for a displacement of 316.4 (5185) instead of the Ae39 data above.
Ae39; G&T (for applications); J1912.
Applications: (UK) Clayton biplane of 1910-1911; Hawkins-Ogilvie triplane of 1911; Sanders biplane No.2, Type 1 of 1911 (built, but not flown); Scottish Aviation biplane of Farman type of 1911.