If you aren't averse to using American parts in your BMW 1600, I have found that a Delco alternator will fit right in the Bosch bracket, and the pulleys will line up.
Here's how I did it on my 1969 BMW 1600. I went to one of my favorite salvage yards and took a 1965 Buick alternator, the type with the separate voltage regulator, as it was considerably cheaper. I took the regulator and wiring harness from regulator to alternator. Use the Delco regulator in place of the Bosch. The wiring hookup is identical to the Bosch; not only is it simple, but it's inexpensive under $20.
Weld the belt tension arm to your BMW tension arm, because they are located a little differently. Also, you have to make a 3/# long spacer or washer stack.
In an hour's time, presto, you're in business, and you pick up about 10 to 15 more amps output, too.
Author: Pete E. Grossenbach
Tech. Ed. Note American alternators are different for six or eight cylinder cars, for sedans, wagons, and air conditioned cars, so it would be helpful to know what model alternator Mr. Grossenbach refers to. This solution is one which should be checked often after installation. Where the alternator tensioning arm carries the entire weight of the alternator, it is subject to breaking where it was welded messy should the arm break. This type of repair is fine as a "stop-gap" but a series of problems like burned water pump bearings, etc., will sometimes result. I'd be curious to know if this solution works for a couple of years without problems. A cheaper and often more reliable way is to seek out a good alternator rebuilder and have him rebuild your unit for $20540.