Toyota Timing Chain
The timing chain connects between the crankshaft-at the bottom of the engine-and the camshaft-at the top-to keep the two operating in synch with each other. As your timing chain wears, it gets loose and begins to slap against the guides.
Eventually the one of the guides breaks, and then the chain starts slapping against the inside of the timing cover. This produces a characteristic rattle. If it continues, the chain eventually cuts through the timing cover and into the back of the water pump area, which causes a major exchange of oil and water that often results in damaged engine bearings.
Alternatively, sometimes the chain will break and fall off the gear that it turns to operate the valves, and the valves will be ruined when they are hit by the pistons. In either case, the cost of the repair goes up significantly. By far, your best course of action is to replace your timing chain prior to that stage of wear. In my early experiences with timing chains, usually when pieces of the guides broke off they fell harmlessly into the engine oil pan and didn’t create any problem. As a result, if the owner was of a mind to, I felt pretty relaxed about watching the wear progress even to the point of the guide breaking entirely off.
However, in the years since I’ve seen that the pieces of guide don’t always fall harmlessly into the engine pan. Sometimes they fall between the crank gear and the timing chain, at which point with each revolution the chain gets hiked over another tooth, and in a very brief moment you’ve got multiple bent valves. In this event, the head has to be overhauled in addition to replacing the timing chain assembly. As a result of seeing this, plus the odd instance when the chain simply breaks, I now feel that it’s far more prudent to take the earliest indications of a worn chain and simply act on that basis to replace it. Otherwise, the longer you wait, the more you increase the odds that the final repair will be as much as two and a half times as much as what it would have been if you’d acted on it more promptly.
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