Question:

 

My daughter has a 1989 Nissan Sentra with about 75,000 miles on it. Recently, it snapped the timing belt. When we took it in for repairs, the mechanic estimated the cost at about $875. He said the pistons smashed up the valves when the timing belt broke. Is this possible, or is the guy just trying to take advantage of us?

 

Answer:

 

I know it seems like a mechanical defect, but it's really not fair to call this a bad design. Many manufacturers use engines in which the valves may be damaged when the timing belt or chain lets go. When the engineers designed the engine they did not plan on having the pistons going up and down without the timing belt attached. And this usually doesn't happen when the belt is replaced according to the manufacturer's maintenance schedule, every 60,000 miles in your case.

 

We know that the timing belt links the camshaft to the crankshaft. Now here's what happened: normally when a piston reaches TDC (Top Dead Center), as far up as it will go, neither the intake nor exhaust valves are fully opened. In other words, the valves aren't pushed down toward the piston as far as they will go. When your timing belt snapped, the camshaft (which controls the opening and closing of the valves) stopped moving, leaving certain valves wide open. Meanwhile, the crankshaft still had a lot of momentum and kept right on pushing those pistons up and down. When the pistons reached TDC, they smashed into the fully-open valves. This collision can bend valves, crack valve guides, damage valve seats and more, depending on the RPM at time of death.

 

There are engines; I call them "Free Wheelers", in which there is clearance between the fully open valves and the piston at TDC. Unfortunately, yours isn't one of them. So don't file charges. The mechanic is probably innocent.

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