Repairing the Timing Chain Guide on Four-Cylinder Engines

A sheared bolt securing a critical piece of engine hardware is never good news, but when that bolt happens to be a failure-prone bolt securing the timing chain guide on General Motors' popular four-cylinder engines, the results can spell disaster. Fortunately, GM engineers made it relatively easy to extract the sheared bolt — and replace it with a more robust one — without disassembling the cylinder head.

The timing chain synchronizes the rotation of the crankshaft with the camshaft(s) allowing the engine to open and close its valves during each cylinder's firing cycle. The chain is lubricated by the engine oil so it must be able to tolerate high temperatures and forces inside the engine. The chain also requires a constant supply of pressure from the timing chain tensioner to keep it slack free and in proper time with the camshaft(s).

A worn or damaged chain, sprockets, or tensioner can cause the engine to misfire or run rough. A faulty timing chain can even knock out the intake and exhaust valves, creating a severe power loss and possible engine damage.

Because the chains, sprockets, and guides are constantly under tension they will stretch over time. When the chain stretches, it can rub against the sprockets and even the guides, which is why regular oil maintenance is so important. The proper engine oil lubricates the chain, sprockets and guides, allowing them to function properly and last for the life of the engine.

When a plastic chain guide becomes cracked or brittle, it can cause the chain to contact and scrape against the camshaft(s) and sprockets, creating a noisy operation and possibly causing them to wear faster than normal. The damaged guide can also block the oil passages, preventing the chain from getting adequate lubrication.

Metal guides were once used on many older engines but have been largely replaced in modern applications with polymer-based materials that are much more durable and can resist the temperatures and forces of an engine. These plastics can withstand thousands of heat cycles and are designed to resist chemicals and harmful vapors from engine blowby.

A good practice is to install new polymer chain guides when replacing the main and accessory chains and sprockets. These will not only improve the operation of the vehicle but will help protect the expensive and delicate aluminum heads of the cylinder blocks. The best part is they are relatively inexpensive, so it just makes sense to do them as you are installing a new chain and sprockets. They are available for most OHC and V6 engines found in passenger vehicles.