I just bought a PowerMaster 140amp alternator from Year One, and the description was as follows:
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1988-1992 5.0/5.7 TPI motors. Simple 3 wire plug connector.
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I have a 1988 Iroc with a 5.0 TPI, and my alternator has the two mounting holes on the lower half about 100 degrees apart. The one that was sent to me has them directly across from eachother. When I called they swore that it was the right alternator. What's going on??

 

send it back say it doesnt fit, sometimes parts get switched up, or the wrong front case plate was installed, it happens. exchange it...year one should have no problem with that...if they do, id never buy from them again

 

A) they could have sent you the wrong part (check the PN stamped on to the part, the box could have been switched)

B) The alternator that you have now along with its mounting brackets could have been added on, either because the previous owner got the wrong alternator or had one lying around and decided to use it. (This is not too likely)

C) It could be that the mounting system you have must be modified to allow the alternator to fit properly.

If the alternator said "direct fit" in the part description, then check the bracketry, and if it don't work, send it back.

 

Well, guess it's back to the old one for a while! What years are your cars, and what does the mounting look like? Mine looks like the attached pic.

 

It is probably the right P/N for your car but the case is clocked wrong. Every time I get an alternator I have to re-clock the case to use it, no biggie.

 

...re clock... you lost me ;)

 

loosen the 4 alt case screws rotate the case to where you need it and if it still dont work swap out the rear case

 

You'll have to remove the 4 bolts/screws that hold the 2 halves together, remove the back half, put the brushes back in the holders and hold them there with toothpicks through the holes in the case (you'll see them) by the brushes. Then put the alt together with the mounting holes in the proper position for your mounts and reinstall the bolts/screws.

 

Re-clocking won't fix this. Usually the mounting holes are both on the same case half, so "re-clocking" won't do anything but orient the plug and any support holes.

Most 140 amp alternators have the holes 180° from each other, but you can still make it fit! Here is an article showing how:
Installing a 140A Alternator

EDIT: The article mentions use of a longer belt, but I have had several people tell me that the stock belt worked fine.

 

Well, I called Year One and they had no idea what was wrong. On the back of the alternator was a paper tag that has the right part # on it, so I was just as confused. I called up PowerMaster, and described it; they told me it is a Corvette alternator (also fits the 454 SS pickup) Shoud I send it back to get a replacement, or is this one worth more than the one I bough, and I shoud sell it?

 

Send it back unless you want to do what GMTech posted. The bolt holes need to be something like 135 degrees apart to fit on the serpentine bracket we have on our cars.