Crankshaft Pulley

The crankshaft pulley or "crank pulley" is the pulley attached to the crankshaft on the front of the engine. This pulley turns a belt which in turn drives accessory items like the power steering pump, air conditioning pump, alternator, and so forth. The crank pulley usually sticks out from the timing belt cover where the crankshaft timing pulley (or gear) turns the timing belt.

What can go wrong with a crank pulley?

A crankshaft pulley can become bent, loose, or cracked in some situations. Most often, the pulley will become bent during removal or re-installation after maintenance on the vehicle such as a timing belt replacement. Some pullers, especially those not made to remove the crank pulley, can damage the pulley in removal.

A cracked pulley often occurs because of over-tightening (over-torque) of the bolt or bolts on the crank pulley. Rust can also cause splitting as can tearing of the rubber on pulleys with a rubberized harmonic dampener installed.

Loose pulleys are usually thanks to under-tightening (not enough torque) of the pulley's bolts or when the wrong pulley is installed and outside of the tolerance levels of the crankshaft and engine manufacturer's recommendations.

After-market pulleys

Most non-OEM, after-market pulleys will be made of steel as are the originals, but may not include a harmonic balancer. These pulleys are usually inferior in quality to the original and may cause problems down the road.

Performance pulleys and underdrive crank pulleys

Crankshaft pulleys made to replace the original and add performance to the vehicle are controversial. While some may increase horsepower or even efficiency, there are usually tradeoffs for this boost.

Underdrive crank pulleys are pulleys that are slightly smaller than OEM and thus turn the belt slightly slower, decreasing the RPMs on the accessories and lowering their torque against the engine. This lowering of the load on the engine allows more power to be used to propel the vehicle, thus increasing performance. The trade is that accessories may have shorter lifespans or not provide enough power to do their jobs properly - alternators may not charge batteries as well, power steering pumps may provide less steering assist, etc.

Aluminum pulleys reduce weight significantly, but do not include harmonic balancing and can cause undue belt wear or slippage because of their differing surface viscosity. They are also prone to cracking in adverse conditions.