De Dion Bouton 70-120-4-31116 Overhead Valve Engine

Despite the mechanical modernization (overhead valves instead of side valves), this engine retains the same robust cylinder architecture that the Puteaux factory used at the time for its mid-size models:

• Engine architecture: 4 cylinders in-line, with overhead valves (OHV) operated via pushrods from the lower block. • Bore × Stroke: 70 mm × 120 mm (designated as 70-120).
• Displacement: 1,847 cc (1.85 liters).

• Fiscal Power: Classified as a 10 CV (French fiscal horsepower), marketed in Great Britain at the time as a 12/24 HP or 12/28 HP.

• Fuel Supply: Original Solex horizontal/downdraft carburetor with a bronze/brass float chamber and a cast iron intake manifold (visible in photos 4 and 5).

• Flywheel: A large, exposed cast iron flywheel at the rear (photo 1) with mounting points for an early dry- or wet-plate clutch. The transmission housing is missing from this specific block, unlike the earlier engine.

1. The Introduction of the Overhead Valve (ca. 1923 – 1925)

In the early 1920s, De Dion-Bouton began losing ground to aggressively priced mass manufacturers such as Citroën and Renault. To defend their status as a premium manufacturer, they introduced modern overhead valve (OHV) engines around 1923. Thanks to this design, the engine breathed significantly better, resulting in higher RPM, more efficient combustion, and more power (approximately 24 to 28 hp) compared to side-valve engines of the same displacement.

2. In which legendary models was this specific OHV block located? A 70×120 mm four-cylinder engine with this specific overhead valve layout was mainly fitted to the more luxurious versions of their 10CV chassis:

• The Dion-Bouton Type IW 10CV (built from late 1922/1923, a model that was frequently equipped with this more modern overhead valve engine)
• The Dion-Bouton Type IT (the more luxurious variant of the well-known Type IS, equipped with four-wheel brakes and often this more advanced engine)

3. Current status in the photos

This block is visually and mechanically a true time capsule. The fact that the Solex carburetor with its copper pipes and aluminium covers is still intact is rare. The engine is currently mounted on a specially welded transport or test stand frame, ready to be restored or demonstrated at idle.