Flathead (Side-Valve Engine)

In a flathead, or side-valve, engine the valves sit in the cylinder block next to the cylinder, pointing upward, and are opened by a camshaft below. The cylinder head is just a flat cover over the combustion chamber, which gives the engine its name.

The layout is simple, compact and cheap, so it was used by many manufacturers in the first half of the 20th century, including Harley-Davidson, Indian, BSA and military bikes. The long, winding gas path limits compression and power, and overhead-valve engines replaced it.

Today flatheads are mostly found on vintage motorcycles and in old small industrial engines. Restorers should watch out for overheating and use oils suited to old engines.