Overhead cams.

[jabartzer]

How do overhead cams work?

[New Forester]

Briefly, the cam shaft controls the opening and shutting of the valves, and it runs by a chain (or nowadays a belt) from the crankshaft. "Overhead valve" means that the crankshaft is in the main engine block, operating push-rods which come up into the cylinder head. Each then operates a rocker, the other end of which works the valve. "Overhead cam" means the camshaft is in the cylinder head itself (the valves are in fact also "overhead"). It works the valves directly, or via rockers. The difference is that an overhead cam is lighter, but also it's easier to adjust. This is because the overhead valve arrangement crosses the cylinder head gasket, which varies slightly in thickness when taking the head off. The OHC has a fixed relationship with the valves, so stays adjusted better. The OHV is so named in contrast to the old side-valve arrangement, where the valves worked upside-down off a camshaft in the block, opening upwards out of the top of the block, next to the pistons. The compression space included a side-chamber around the valves, and the cylinder head was pretty much a flat slab without gubbins. Cheap to make, but inefficent. Some old mowers may still be found like this, and of course older cars (generally pre-1960 in the UK).