Good Timing

Timing belts and timing chains both serve the same purpose: to drive the camshaft. Is one approach better than the other?

The camshaft in an internal combustion engine is driven by the crankshaft using one of three methods—direct gear drive, sprockets and timing chain, or sprockets and timing belt. Rarely do you see an engine these days with direct drive timing gears, but sprockets driven by chains and belts are very common.

For many decades, timing gears and timing chains were the method of choice. However, when overhead cam (OHC) engines began to become popular in the late 1970s, timing belts, made from composite materials, were the preferred choice. Recently, though, the timing chain, which had lost favor in many applications, has found new life and is becoming the norm rather than the exception.

Belts vs. Chains

Fundamentally, a timing belt and a timing chain perform the same task—driving the camshaft. However, from a construction standpoint, the two are quite different.

A timing belt is a simple device, made from a flexible, composite material. This material is lightweight compared to the chain and can be stretched. A combination of high-strength and high-temperature neoprene provides strength with the desired service life. The timing belt is flat on one side with evenly-spaced cogs, or teeth, on the other. These cogs fit snugly into grooves on the timing sprocket.

The weight advantage that the timing belt has over the timing chain is its major advantage. Timing belts are used with overhead cam (OHC) engines. These engines do not have pushrods and traditional valve lifters. With this weight removed from the valvetrain, along with the weight of a chain, the valvetrain has significantly less inertia, which enhances the performance of the valvetrain. A typical timing belt application is illustrated in Figure 1, the 2.4L engine used in the PT Cruiser.

On the negative side, timing belts have a definite service life, usually about 100,000 miles. Now that’s fine if you’re buying a new car every three years or so, but the average age of a passenger car is almost ten years, so, somewhere along the line, that belt is going to have to be replaced.

The timing belts from the early OHC engines (late 1970s to early ’80s) lasted about 60,000 miles. Belt failure was rather common until the higher grade formulations of neoprene were introduced. The problem with belt failure is that some engines are not free spinning. A free spinning engine is one in which the valves will not interfere with the pistons if the belt fails, but if the engine is not such a design, catastrophic engine damage can occur.

Timing chains, on the other hand, rarely break and usually last the life of the vehicle. Made from steel, a typical timing chain is a series of roller links (inside links of the chain that consists of two bushed rollers connected by two plates) joined by two outer plates. Due to the size of the outer plates, the bushed rollers are equally spaced along the entire length of the chain. The teeth of the timing sprockets fit into these spaces. A typical timing chain application is seen in Figure 2, the 3.3/3.8L V6 engine used in the minivans.

The major advantage the chain over the timing belt is it strength. As we mentioned, timing chains rarely break; however, there are some disadvantages, most notably weight and noise. Valvetrain inertia increases with a timing chain and with higher revving engines this can present a problem if the valvetrain is not properly designed.

Chains can also be noisy. Noise occurs every time a link rolls onto a sprocket tooth and every time a chain link bends. The chain in Figure 2 has 32 links and 64 pins. As the speed of the engine increases, this noise increases. As the chain wears, it can begin to hit on the inside of the timing chain, increasing the noise level. The primary solution to the noise problem is the use of composite-lined chain guides that absorb chain noise and keep the chain properly aligned, preventing the chain from contacting the inside of the cover.

It appears that chains have become the dominant valvetrain drive mechanism. Strength and durability seem to have won out over weight and flexibility in many applications. As you can see, though, regardless of the drive mechanism chosen, there are pros and cons to each. Let’s examine some basic setup procedures for the chain and belt to ensure maximum performance.

Timing Chains

When installing timing chains, it is critically important that the timing marks on the camshaft and crankshaft sprockets are aligned, as shown in Figure 2. Even if the marks are off one tooth, engine timing will be significantly altered. Use a straightedge to check the alignment.

Some people think that timing chains stretch, which can lead to problems, especially increased noise levels. What’s really happening is that the chain has worn (don’t forget the wear on the cam and crank sprockets). If you could add up the wear in each pin over the length of the chain, the chain is longer, giving the impression that it has stretched. This wear is something that should be checked, especially if the engine has seen significant service(exceeding 100,000 miles).

To check wear, position a scale next to the timing chain so that any movement of the chain can be measured (see Figure 3). Place a torque wrench and socket on the camshaft attaching bolt. Apply force (30 lb-ft with the cylinder heads installed) in the direction of crankshaft rotation to take up any slack. Next, block the crankshaft so it cannot rotate, then apply the same force in the opposite direction. Measure the amount of sprocket/chain movement. If movement exceeds 1/8" (.125"), replace the timing chain and sprockets.

Timing Belts

As mentioned previously, timing belts can stretch. As a result, a timing belt must be properly tensioned when it is installed. A loose timing belt can cause the teeth to be sheared off the belt. Let’s review this critical procedure. We’ll use the 2.4L DOHC engine in our example.

Install the timing belt in the following manner:

1. Set the crank sprocket to TDC as shown in Figure 1.
2. Set the camshaft timing marks so that the exhaust cam sprocket is half a notch below the intake cam sprocket (as you look at the engine, the exhaust cam sprocket is on the left).
3. Install the timing belt; start at the crank sprocket and go around the water pump sprocket and continue in that direction.
4. Rotate the exhaust cam sprocket counterclockwise to align the marks and take up slack.
5. Insert a 6 mm Allen wrench into the hexagon opening in the top plate of the belt tensioner pulley (see Figure 4). Rotate the top plate counterclockwise to move the tensioner pulley against the timing belt. The tensioner setting notch will move clockwise.
6. While watching the movement of the tensioner notch, continue rotating the top plate counterclockwise until the setting notch is aligned with the spring tang (see Figure 4).
7. Using the Allen wrench to hold the top plate in place, torque the tensioner lock bolt to 220 lb-in. The setting notch and spring tang should remain aligned after the lock nut is torqued.