An easy way to see if you have a large or small case alternator is to check the holes on the front, near the pulley. This is called the drive end, and on small case alternators there are four small holes per each of the three ribs radiating from the center. On the large case, there are only two holes on the same ribs. Below we have the large case overlayed on a small case. The upper left of the image is the large case and the holes are highlighted red. The lower right shows the small case with its holes highlighted blue.
Stock small case 3G units usually had an output of about 90 amps, the large case puts out about 130 amps. Our 3G series large case alternators start at 130 amps and go up to 250 amps with our Quicktifier Dual and Triple Rectifier System. Click here to order our 3G series large case alternator.