How to Check Wear in an Engine’s Timing Chain
Since you lack X-ray vision, you employ other methods to uncover issues. When it comes to crops, you may dig a root pit, perform soil analysis, or pull tissue samples to uncover what is going on within a plant or below the ground. Likewise, when it comes to the timing chain in an engine, there is an accurate method to determine its wear without taking the engine apart.
Since the camshaft and the timing of the valve events are controlled by the rotation of the crankshaft and often linked by a chain, any wear will affect not only the valve action but also the timing of the ignition through the distributor.
The stackup created by stretch in the timing chain can present many different ways. The first sign is a decrease in engine performance. As the chain grows longer, there is the possibility of it actually wearing through the timing cover. If the chain tension skews enough, it can jump a few teeth on the timing gear and, at best, the engine will stall. However, the piston can collide with the valves to potentially ruin the block and cylinder head. In some cases, the chain will make a knocking noise if it is extremely stretched – but don’t count on that warning.
As the timing chain wears, the position of the camshaft in relation to the centerline of the intake lobe retards. The distributor is run via the gear on the camshaft, so the position of the distributor shaft and primary trigger (breaker points or reluctor ring with electronic ignition) will change and retard the ignition timing, too.
A first sign of a timing chain with excessive slack is a change in ignition timing with no change in the position of the distributor housing. For example, if you set the timing at 10° before top dead center (BTDC) and the distributor hold-down is tight and over a period the timing is now at 6° BTDC, the most likely cause is slack in the timing chain.
You can then just reset the ignition timing to the proper specification, but you need to keep in mind that there is wear in the chain.
Over the years, I’ve seen many engines that were junked due to a worn timing chain. Some had a rhythmic knocking sound that was misdiagnosed as a rod bearing or a lack of power interpreted as a worn engine. For this reason, you should never jump to conclusions when diagnosing any piece of equipment – especially an engine.
To accurately identify the amount of wear in a timing chain, begin by removing all of the spark plugs and the distributor cap. With a wrench on the harmonic balancer bolt, turn the crankshaft at least two complete revolutions backward of rotation to remove any slack from the timing chain.
While you are still turning the crankshaft opposite to its running rotation, bring the timing mark on the harmonic balancer to top dead center (TDC) on the timing tab or pointer. With a helper watching the distributor rotor, gently turn the crankshaft in the proper direction of rotation.
As soon as the rotor tip starts to move, STOP! Read the amount of stretch on the timing tab.
For example, if the rotor tip began to move at 10° after TDC as measured at the timing mark, the chain has 10° of stretch. Any reading more than 4° to 5° is considered excessive and you need to schedule time to install a new timing chain and gear set.
As an aside to this, many early General Motor light-duty diesel engines used in pickup trucks foolishly employed a timing chain instead of a gear train to run the camshaft and the injection pump, since the engines were based on the gasoline versions. Due to the rotational load on the chain, it was common for excessive stretch to occur over time. Diesel-injection pump timing is not as forgiving as the distributor on a gas engine. The result was a poor running engine, but few knew it was caused by a worn timing chain.
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