Timing Belt versus a Serpentine belt — What’s the difference

Timing belt versus a Serpentine Belt — Different Belts for Different Jobs

People often confuse a timing belt versus a serpentine belt. They’re both belts. They both get their rotational force from the crankshaft and use that force to drive other components. But that’s where the similarities end. A serpentine belt’s primary purpose is to rotate driven components like the alternator, AC compressor, Power steering pump, etc. Those driven components don’t have to be driven in time with the crankshaft. If a serpentine belt slips, there’s no harm, no foul.

A timing belt’s primary job is to synchronize the rotation of the crankshaft with the camshaft. If the timing belt slips or fails, the engine can be driven out of time and destroyed.

A serpentine belt is really just a series of small V shapes with a fabric backing that can drive components from the backside of the belt. But a timing belt has molded cogs that fit into corresponding teeth on the camshaft and crankshaft sprockets. Once the crankshaft and camshaft sprockets and timing belt are aligned, they must stay that way until it’s time to replace the belt. The cogs prevent the belt from slipping out of time with the driven components and keep the camshafts synchronized with the crankshaft. So there’s really no comparison between a timing belt versus  a serpentine belt as far as what they do.

Why carmakers use timing belts

Back in the days when most engines used pushrods to activate the intake and exhaust valves, carmakers used timing gears or a timing chain to drive a single camshaft which moved the pushrods. When they switched to overhead camshaft engines, the distance from the crankshaft was considered too great to use a timing chain, so they switched to a timing belt (see the graphic below).

The upside to a timing belt is that they’re quiet and cheap. Unfortunately, they wear out and must be replaced. And that can cost upwards of $1,500 on some engines.

You can’t see the timing belt

Timing belts are always covered to prevent them from being contaminated by oil or coolant and keep them free of road dust, dirt and grite.

Here you see a Subaru engine showing the timing belt covered and uncovered. In real life, you would never see the timing belt uncovered

Here is what confuses people. Same Subaru engine but with the serpentine belt routed around all the driven components. But the timing belt is cover up.

Timing Belt types and routing

Timing Belt Construction — Cog Profiles

Timing belt cog profiles differ based on the pulley diameter and rotational force required.

©, 2022 Rick Muscoplat