Tighter is not better when it comes to bolts

Dear Car Talk: I have a 2010 Honda Pilot. Last week, the bolt that goes through the pulley holding the tensioner assembly sheared, and the belt did not break. I was able to remove the remaining bolt and installed a new assembly. Three days later, the bolt sheared again and the belt did not break. What would cause this? Thanks. — Wesley

You did, Wesley. I’m guessing you over tightened the bolt.

It’s pretty common for this bolt to break on this vehicle. I don’t know why — a rare case of underdesigning a part by Honda. So, the fact that it broke after 12 years is not unusual or alarming.

But my guess is that when you replaced it, you tightened the bejeebers out of it. The tighter, the better, right? Except this bolt calls for a torque of only 40 foot-pounds or so. That’s not a lot. And if you tighten it with your socket wrench, it’s very easy to go a lot tighter than that. And what happens when you over tighten a bolt? It stretches, making the metal more brittle, and prone to breaking. We see this with wheel bolts, where it can be very dangerous. It’s easy to assume that when you’re attaching a wheel, the tighter the better. But if the wheel bolts get over tightened and stretched, they can snap off while you’re driving.

And if you think it was exciting having your tensioner assembly bolt shear off, try five wheel bolts on a curve at 50 mph.

Anyway, I’d try one more time. And this time, use a torque wrench. Or have a mechanic do it for you if that’s not in your toolbox or your skill set.

I think you’ll have better luck. And if not, don’t write back, Wesley, because that’s my best guess.

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