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是看不见的
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原文：
Oil-Filter Magnets Don't Hurt, But Don't Help A Lot
Posted to: Car Talk
169; April 25, 2007
By Tom and Ray Magliozzi
Dear Tom and Ray:
I recently saw a car catalog that had an oil-filter magnet for sale. The magnet was placed on the bottom of an oil filter and was strong enough to remove any metal shavings from the oil stream. Is this product worthwhile, or am I just wasting my money? --173; Ron
TOM: These things have been around for years, Ron. They certainly don't do any harm.
RAY: They sell magnets that go inside the oil pan, magnets that are built into the oil plug, and magnets, like this one, that attach to the filter.
TOM: Now, keep in mind that the oil filter itself already removes any metal shavings, or anything else that's bigger than about 25 microns 173; or about half the width of a human hair. So they do a pretty good job.
RAY: Does stuff that's smaller than that 173; stuff that gets through the filter 173; harm your engine? Probably, at least a little. But with the magnet, you'll only get the metallic stuff that sneaks by the filter. You won't catch any soot or pieces of aluminum, which are also harmful.
TOM: If a magnet really made engines last longer, don't you think that GM and Ford would already have spent the three cents per car it would take to build one into the oil plug?
RAY: So I'd have to say that oil-filter magnets fall into the general category of "overkill."
TOM: But if you're the kind of guy who believes in overkill, Ron, then go for it. Do you wear a surgical mask on an airplane? Do you go back and double-check to make sure you've turned off the stove before you leave the house? Do you have your brother taste your food before you eat it? If so, you should get one of these magnets, Ron.
RAY: In fact, you should get two, in case one falls off.
TOM: Seriously, it certainly can't hurt anything. And it's possible that there's some small benefit. But in the big scheme of things, it's not high on the priorities list. Changing your oil regularly will probably extend the life of your engine more than any magnet.
原文links：http://hamptonroads.com/node/256401_(a) _(a) _(a)
