A flathead engine, also known as a sidevalve engine or valve-in-block engine, is an internal combustion engine with its poppet valves contained within the engine block, instead of in the cylinder head, as in an overhead valve engine.
A crossflow T-head sidevalve engine
1915 Cadillac flathead engine block
Harley-Davidson flathead
BMW R12
A poppet valve is a valve typically used to control the timing and quantity of petrol (gas) or vapour flow into or out of an engine, but with many other applications.
Overhead camshaft engine
Oscillating poppet valve on one of Chapelon's rebuilt 4-6-2 locomotives.
Image: Poppetvalve