Section: 3/32" (2.4mm)
Diameter of the base of the groove is 77mm
Accepting bore is 81mm

The O-ring seal on the alternator is not under great pressure, but it does tend to swell with age. When the alternator is removed, you may notice that the o-ring is quite slack in it's groove. If it is replaced in that state, chances are that the bore will cut it, as it slides back into position. The resulting oil leak is likely to be very small, but annoying - it took 2 days to make it's presence known (-;

The solution to the problem is to cut about 20mm out of the o-ring, and rejoin the ends with super-glue. When doing this, use a very sharp knife - a new scalpel is preferable - to ensure a clean, square cut. Slight mis-alignment is acceptable.

Also, when refitting, a small drop of superglue will hold the flexible couplings in place, long enough, so that they don't mis-align.