Alternator off set question?

[126coupe]

I have an alternator housing w a 901 part #. My new alternator will not work. The bolts that attach the Alt. to the housing only show a few threads. Did Porsche change the off set often or do I need a later Alternator housing??

Thanks in advance

[kodioneill]

i have a 74 911 targa. my alternator failed it was a marcal. i replaced it with a bosh and had the same problem. i measured the offset and took it to a machine shop and they turned the housing on a lathe to remove about 7mm from the relief area where the base of the alternator rests. it worked perfect. if you google (74 alternator porsche offset i believe you will find others with the same delemma.) also after install the alt light would not go out unless i revved up to about 2500 rpm. i had to install a resistor across the bulb terminals at the indicator light to give enough resistance for the alternator to excite the field.
i don't recall the value of the resistor. the problem is the bulb wattage. it must be 3 watts. a 3 watt bulb does not fit in the tach housing.

[Early_S_Man]

The 1975-production housings were machined 9.3 mm deeper than the 1974 housings. The interim engines got a spacer ring and 840 Watt alternator [same size as previous 770 Watt alternator housing] until the 980 Watt Marchal alternator became available.