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I recently took a vehicle (2008 Sedona Automatic Transmission) in to a transmission place on the advice of a basic auto repair guy, because the metal grinding noise we were hearing, he believed, was coming from the transmission.

At the transmission place, they guy said, no, it turns out it's the engine.

After further investigation he told me the engine would have to be replaced. This effectively totals the car.

That's all okay. Here's my question.

I don't remember what the transmission/engine guy said word for word, but what I took away from his description was a mental image of the piston in the engine literally hitting or scraping against the side of the little chamber it sits in in the engine. That, I thought I understood, was the source of the metal grinding sound (according to his description), and the damage already done by this (and the damage that caused it, whatever that was) necessitated a complete replacement.

However on relaying this mental image to someone casually who generally "knows about cars," he laughed and said "that's not how that works." There was no further elaboration.

I'm prepared to believe I either misunderstood the engine/transmission guy, or he massively oversimplified it in describing it to me.

But my question is: If I say to you "my engine was making a metal grinding sound, and it turns out it was a result of the piston making contact with parts of the engine it was not supposed to be making contact with," is that an automatic "heh no that's not how that works" from you as well? If so, what do you think was the likely true story behind whatever oversimplification I heard or misheard?

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Not hearing the car myself, the guy who said, "that's not how it works" is pretty much spot on to me. The only way you are going to hear metal-on-metal noise coming from the pistons is if it has run out of oil, and then once the noise starts, the engine will usually seize in a short period of time and you won't be hearing anything from the engine anymore. This is probably true of any piston noises except for slap. Piston slap is when the cylinder bore gets a little big and the piston actually starts hitting the bore during the combustion stroke. It is usually hard to discern from a rod knock and is not a metal grinding noise.

The reason for the noise aspect of it is because the pistons are usually well supported in the bore. You have a cylinder of metal, then another piece of cylindrical metal (the piston). The piston is only slightly smaller (thousands of an inch difference) than the cylinder bore. Then you have oil which cushions the piston in the bore. There is a lot there to support it, so noise like you're describing doesn't happen. (Hopefully that makes sense.)

I'm thinking this is a case of over-simplification and maybe not really knowing what is going on (so, an assumption from the mechanic). Some noises are very hard to diagnose, so a lot of it is just a guess. The only thing I can think of which might make a metal-on-metal noise would be if a bearing has completely lost lubrication. The only thing there is, an engine part without lubrication would not last very long. I don't know how much the vehicle has been driven (if at all), so there's that.

If there is any single thing internal to the engine which a metal-on-metal noise might indicate, the only thing which comes to mind is a bearing (be it main, rod, or cam). What may be happening is the mechanic who looked at it is actually fooled as to where the noise is coming from. It may be external of the engine, but coming from the engine bay, such as a water pump or an alternator. Those can and will make metal-on-metal noises when they are completely worn out.

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