Bearings seals do matter. They keep out dust (there is large amount of air being pulled through the alternator with accompanying dust) they also retain the grease in the bearing.
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What I should have said is that just about any bearing you buy, unless you special order it, will have some kind of seal. The suffix says what kind of seal, if both sides are sealed, etc. You should have some seal, not only to keep the dust out, it also keeps the lubrication in. The way the bearings are located inside the housings you won't get a whole lot of air through the bearings. In fact, because of where the alternator is located you don't get enough air through it which is part of the problem. Makes it run hotter than it should. Thanks for the correction.
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