Valves are critical to the four strokes of an engine. They’re what let the intake mixture of air and fuel — or just air, if it mixes with fuel inside the combustion chamber when direct-injection is present — into the engine, then, once it’s burned, let out its exhaust gas byproduct. This segment of the engine is called the valvetrain.
Another crucial part of the valvetrain is the tappets, also known as buckets, followers, or — also common in automotive parlance — lifters. The prime example is a single-camshaft (or, cam for short) engine, like the GM LS V8. As the camshaft spins, the tappet is pushed up, which pushes on the pushrod, which then actuates the rocker, which finally actuates the valve.
When it comes to the cam, you may have heard two different types; what’s the difference between roller and flat tappet cams? Let’s discuss both, as well as the design of each tappet that they actuate.
Flat tappet cams are the most common and widely used, especially from the factory floor. The tappets are, well, flatter, and the camshaft’s lobe, the bit that contacts the tappet, is more pointy. Similar to what you may have seen in an overhead-cam engine.
We say flatter because these tappets aren’t completely flat, but instead have more of a crown shape on the surface that contacts the lobe. As the lobe’s slight taper pushes on them, they spin, which cuts down on wear between the two surfaces. Add in the fact that they’re always (or at least, should be) receiving oil, and they last a good, long while.
Because these tappets are simpler than roller tappets, they’re cheaper to produce. If someone replaces their camshaft, they should replace the tappets as well as they break-in/wear together. A flat tappet situation works well for most folks’ needs and can ensure a more budget-friendly aftermarket build. Modern technology has also enabled flat-tappet setups to make around the same power as roller-tappet, too, but since they wear faster, one should consider pouring in a zinc additive (or zinc-rich oil) to cut down on this.
Roller cams are fractionally more complex in the sense that their tappets feature rollers. There’s far less friction between the roller and the camshaft lobe, and allows for more precision valve timing control. The cam’s lobe surface is perfectly flat, and the tappets do not spin as they move up and down, unlike a flat tappet setup. Well, it’s flat in the sense that it’s much more rounded than the pointier than its flat-tappet counterpart.
Because there’s a roller present, plus ample lubrication, these two components don’t wear together, so one can simply swap individual tappets out as needed, and can skip replacing the lot when swapping in a new/different roller cam.
Then, roller cams allow for steeper lobes, which not only opens the valves faster, but gives them more duration, meaning, they stay open longer, which are part of camshafts’ basic specs. This means the engine takes in more air and pushes out more exhaust, leading to more power output. Additionally, the efficiency of the roller setup itself allows for more power.
The downside of roller cams and tappets is they’re more expensive, but easily allow for more performance, even if they’re slightly heavier. These tappets also must be anchored to prevent spinning — if they do, it could cause major engine damage. Finally, they’re more complex and necessitate changing up and/or adding more components to make them work where flat tappets originally lived.
The difference between roller and flat tappet cams, then, lies in their appearance, design, and the amount of friction going on. There are cost, complexity, and performance differences, too.