Dear Car Talk:
You recently wrote about a guy whose timing belt broke, and who tried to restart his car but whose valves survived only through sheer good luck. In 1957, I had a new Ford six-cylinder, standard shift. Driving out in the country late one night, when the car was relatively new, the timing belt broke. There was no damage to the engine, but I did try to crank it several times, not knowing what had caused the initial loss of all engine power.
Was I also one of the lucky ones, or was there something different about that model of Ford? -- Lindle
There are a lot of differences between the 1991 Honda Civic I wrote about and your 1957 Ford, Lindle. One is that a 1991 Honda was extremely unlikely to die on you in 1991. But more importantly, your old Ford did not have an "interference engine."
Starting in the 1980s, Honda, and lots of other manufacturers, started making engines in which the open valves and the pistons shared the same space at times -- or overlapped -- inside each cylinder. The timing belt, as its name suggests, ensures that when the valves drop down from the top and open into the cylinder, the piston is not near the top at the top of its stroke. And when the piston comes up to the top, it ensures that the valves are closed and out of the way.
The advantage of this design is that it allows for an increase in compression ratio and lets the valves get wider, which means more power and better mileage from the same-size engine.
But if the timing belt breaks or jumps, the pistons can -- and often do -- crash into the valves and bend them. That's why manufacturers -- especially those that use interference engine designs -- insist that customers change their timing belts at 90,000 miles (on average). And why we strongly reinforce that advice to our customers.
Your Ford did not have an interference engine. So the valves could be fully open and the piston could be at the top dead center, and the two would not touch. So among the many problems you probably had with that Ford, getting the valves crushed by the pistons was not one of them.
You also didn't have a timing belt. You had a timing chain, or even timing gears, which were more common then.Interestingly, we're seeing a lot more timing chains again these days, as manufacturers have figured out how to make them truly reliable (unlike the one in your 1957 Ford). And now they almost never fail.
So you were one of the lucky ones, Lindle. Lucky you were driving a '57 Ford instead of a '07 Ford.