A valve train is the system of mechanical components inside an internal combustion engine that opens and closes the intake and exhaust valves. Every time your engine runs, these valves must open to let air and fuel in, then close to seal the combustion chamber, then open again to release exhaust gases. The valve train controls this entire sequence with precise timing, thousands of times per minute.
What the Valve Train Does
An engine cylinder needs to breathe. During each combustion cycle, air and fuel must enter the cylinder, combust, and then the spent gases must exit. The valves act as gates controlling this flow, and the valve train is everything that makes those gates open and shut at exactly the right moment.
If the timing is off by even a few degrees of crankshaft rotation, the engine loses power, runs rough, or in severe cases, a piston can collide with an open valve and cause serious internal damage. The valve train translates the rotational energy of the engine into the precise up-and-down motion of each valve, synchronized perfectly with piston movement.
Components of the Valve Train
The valve train includes every part between the camshaft and the valve itself. While exact configurations vary by engine design, the core components are consistent across most engines.
- Camshaft: A rotating shaft with egg-shaped lobes machined along its length. Each lobe corresponds to one valve. As the camshaft spins, the lobe’s profile pushes against the next component in the chain, dictating exactly when the valve opens, how far it opens, and how long it stays open.
- Lifters (tappets): Small cylindrical components that ride on the camshaft lobes and transfer the lobe’s motion upward. In some engines, these are hydraulic and self-adjusting, using oil pressure to eliminate small gaps in the system. In others, they’re solid and require periodic manual adjustment.
- Pushrods: Long, thin metal rods found only in overhead valve (OHV) engines. They carry motion from a camshaft located in the engine block up to the rocker arms in the cylinder head. Overhead cam engines don’t use pushrods because the camshaft sits directly above the valves.
- Rocker arms: Pivot-mounted levers that reverse the direction of motion. One end gets pushed up by the pushrod or camshaft lobe, and the other end pushes the valve down to open it.
- Valves: The valves themselves, typically made of heat-resistant steel alloys. Intake valves let the air-fuel mixture into the cylinder. Exhaust valves release burned gases. Most modern engines have four valves per cylinder (two intake, two exhaust) for better airflow.
- Valve springs: Coil springs that snap each valve back to its closed position once the camshaft lobe rotates past. Spring tension must be strong enough to close the valve quickly but not so strong that it creates excessive friction and wear on the cam lobes.
- Timing chain or belt: Connects the crankshaft to the camshaft, ensuring the valves open and close in sync with piston position. The camshaft spins at exactly half the speed of the crankshaft in a four-stroke engine, meaning each valve opens once every two full engine revolutions.
OHV vs. OHC Designs
The two main valve train layouts differ in where the camshaft sits, and this single difference affects everything from engine size to performance characteristics.
In an overhead valve (OHV) or “pushrod” engine, the camshaft is located low in the engine block, near the crankshaft. Motion travels upward through lifters, pushrods, and rocker arms to reach the valves in the cylinder head. This design is compact in width, which is why it remains popular in trucks and V8 engines where underhood space is tight. The trade-off is that all those extra moving parts add weight to the valve train, limiting how fast the engine can rev before the components can’t keep up. General Motors’ small-block V8 is one of the most well-known pushrod engines and has been in production in various forms for decades.
In an overhead cam (OHC) engine, the camshaft moves up into the cylinder head itself, sitting directly above the valves. This eliminates pushrods entirely and often reduces or eliminates rocker arms. With fewer parts between the cam and the valve, the system is lighter and can operate at higher engine speeds. A single overhead cam (SOHC) engine uses one camshaft per cylinder head, while a dual overhead cam (DOHC) engine uses two: one for intake valves and one for exhaust valves. Most modern passenger cars use one of these OHC configurations.
Variable Valve Timing
A fixed camshaft profile is always a compromise. A cam lobe designed for smooth idling won’t deliver peak power at high RPM, and a lobe optimized for high-RPM performance will make the engine run rough at low speeds. Variable valve timing (VVT) systems solve this by adjusting when the valves open relative to piston position, and sometimes how far they open.
Most VVT systems work by rotating the camshaft slightly ahead or behind its default position using oil pressure. At low speeds, the timing favors fuel efficiency and smooth operation. As the engine revs higher, the system advances or retards the camshaft to improve airflow and power output. Honda’s VTEC system takes this further by switching between two different cam lobe profiles at a set RPM threshold, essentially giving the engine two different personalities. Toyota, BMW, and most other manufacturers have their own versions of variable valve timing, and the technology is now standard on virtually all new engines.
Some advanced systems can also vary valve lift (how far the valve opens) and duration (how long it stays open) independently. A few engines can even deactivate individual cylinders by keeping their valves closed, improving fuel economy during light cruising by making a V8 function as a V4.
Common Valve Train Problems
Because the valve train has so many moving parts operating at high speed, wear and failure show up in predictable ways.
A ticking or tapping noise from the top of the engine often points to excessive clearance in the valve train, sometimes called “valve lash.” In engines with hydraulic lifters, this can mean a lifter has lost oil pressure and collapsed slightly, creating a gap. The ticking is the sound of metal slapping across that gap thousands of times per minute. In engines with solid lifters, the clearance naturally increases as components wear and requires periodic adjustment.
Timing chain stretch is another common issue in higher-mileage engines. As the chain elongates over time, valve timing drifts from its intended setting, causing reduced power, poor fuel economy, and sometimes a check engine light related to camshaft timing correlation. Timing belts, used in some OHC engines instead of chains, don’t stretch but can snap if not replaced at their service interval, typically between 60,000 and 100,000 miles. In an “interference” engine, a broken timing belt almost always results in piston-to-valve contact and significant internal damage.
Worn cam lobes are less common but happen in engines with insufficient lubrication or contaminated oil. As the lobe wears flat, the valve opens less than it should, starving that cylinder of airflow. The result is a misfire or noticeably uneven idle. This type of wear is usually not repairable without replacing the camshaft.
Why Valve Train Design Matters
The valve train is one of the biggest factors determining an engine’s character. A pushrod V8 with a mild cam profile will produce strong low-end torque and a deep exhaust note. A DOHC four-cylinder with variable valve timing will prioritize efficiency at low speeds and power at high RPM. Aftermarket camshaft upgrades are one of the most popular performance modifications precisely because changing the cam lobe profile reshapes the entire power curve of an engine.
For everyday drivers, the valve train mostly works in the background. Keeping up with oil changes is the single most important thing you can do to protect it, since every component in the system depends on a steady supply of clean oil for lubrication and, in many cases, hydraulic operation. Most valve train components are designed to last the life of the engine when properly maintained.