Pistons traveling up and down inside an engine are only half of the internal combustion story. While the pistons are responsible for the grunt work of spinning the crankshaft, it is the intake and exhaust valves that allow each one of the cycles crucial to the operation of a four-stroke engine to occur.
The intake valve opens on the intake stroke. The piston draws air and fuel into the cylinder as it travels downward. The intake valve then closes. The piston moves up to compress the mixture on the compression stroke. The spark plug ignites the mixture at the very top of piston travel. As the confined fuel-air mixture burns, it pushes the piston back down for the power stroke. The exhaust valve then opens. The piston pushes the spent gas out of the cylinder for the exhaust stroke. In this way the internal combustion creates power using the Otto cycle, so named for German engineer and inventor of the internal combustion engine, Nicolaus Otto.
Half speed bumpstick
Valve timing is of great importance for the Otto-cycle engine to run smoothly and produce power to turn the drive wheels. The intake and exhaust valves in the cylinder head open and close in time by way of one or more camshafts. The camshaft is also aptly referred to in some circles as a bumpstick. On the camshaft are lobes, or circular ramps. The lobes are responsible for opening and closing the valves at precisely the right moments in the never-ending Otto-cycle. These lobes mechanically bump the valves open and allow them to close. Springs hold the valves closed in the cylinder head when not getting bumped around by the camshaft lobes. The camshaft spins at exactly half the revolutions of the engine crankshaft. Gears connected to the camshaft and crankshaft are joined together by the timing chain or belt. If all is well, perfect valve timing is the result.
Key to combustion
Timing chains used to be much more common than timing belts. While this balance has shifted, manufacturers still use timing chains in modern engines. An engine with a worn and slacked timing chain will run poorly, because correct valve and engine timing cannot be achieved. An engine with a broken timing chain or belt will not run at all.
Replacing the timing chain is a major job, as it involves traveling down to the core of the engine itself. A service manual is the first and most important tool on the list for timing chain removal and replacement. Timing chain replacement complexity depends on engine configuration and number of camshafts. That said, engineers build relatively foolproof methods of lining up the gears and chains into the engine design itself.
How to replace a timing chain
The following is an example of the simplest of timing chains on a single camshaft pushrod four-cylinder engine.
Step 1: Getting down to the timing chains first requires removal of everything in the way of the timing chain cover. This includes – but is not limited to – radiators, air-conditioning compressors and alternators. Set the engine to Top Dead Center, and use an impact wrench to remove the crankshaft pulley bolt and pulley remover to remove the pulley itself.
Step 2: Remove all the bolts holding on the timing cover. Removal of the oil pan may be required. If the cover remains stubborn, use a rubber mallet to tap loose the gasket’s grip on the engine block. Tip: Timing cover bolts are of specific lengths. Keep them in their respective holes for easy reassembly.
Step 3 : Here the engine is at Top Dead Center. Note bright links on the timing chain. The bright links line up with corresponding marks on the crankshaft and camshaft gears when the engine is at Top Dead Center. Keep the engine at Top Dead Center while removing and installing timing chains.
Step 4 : Remove the camshaft bolt, making sure not to move the camshaft or engine from Top Dead Center. Impact wrenches work very well for removing bolts. Remove the timing chain set as a unit. Be careful not to lose the crankshaft keys.
Step 5: This timing chain uses a tensioner and guide. Tensioners often operate via oil pressure and may require pre-oiling. Use a bit of bailing wire to hold the tensioner in place while installing the chain. Note the guide pin position on the camshaft and crankshaft key position at 12 o’clock.
Step 6: Align the gear timing marks and bright links on the new timing set before installation. Install the new timing set and button everything back up. Use a new crankshaft seal and new gaskets. Install all bolts to correct torque values. Model-specific information and torque values are in the vehicle service manual.