How Are Four-Valve Cylinders Designed?

A brief description of the roles the camshaft, tappet, spring, and valve play in the combustion process

This cylinder head is from a Honda CBR600RR. It is a four-valve-per-cylinder engine, so with four cylinders it has 16 valves. The intake valves are slightly larger than the exhaust valves located on the opposite side of the combustion chamber because exhaust gas has pressure to push it out; intakes have only the atmosphere to push the charge into the cylinder.

I’ve removed the four valves from one cylinder. The valves are operated by a camshaft. The cam lobe—one of eight—rides against an inverted bucket tappet. It is open on one end, and the wear surface on this example is beautiful and unworn after 25,000 miles, which shows you how wonderful oil is.

As the camshaft revolves, it pushes the tappet and the valve opens to admit a fresh charge of fuel and air to the cylinder. Each valve has a spring, which makes sure that the tappet, valve, and retainer follow the camshaft and the valve is held shut. Is this case, there are two springs nested together that sit around the valve.

At peak rpm, each valve is opening and closing 113 times per second. The whole process of initially lifting the valve, accelerating it toward open, decelerating it to a stop at the top of its lift, and then reversing its motion—it’s now being driven toward closed by the spring following the cam lobe—takes 0.003 of a second, again at peak rpm.

Because valves are exposed to combustion heat, they expand as the engine warms. So there must be clearance between the tip of the valve, the stack of mechanism, and the cam lobe. Otherwise, as the valve expands, it would force itself open. When people say they have to get their valves “clearanced,” it means restoring clearance to accommodate thermal expansion.

Resetting the clearance to nominal values is done with tiny clearance buttons, a little piece of hard steel ground to a specific thickness. I won’t go into the process here, but it is very sensible. That process is also time consuming.