O-ring for sealing the distributor body to the distributor base
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
This O-ring is used to seal the distributor body to the distributor base. The groove in the distributor body has a diameter of 21 mm and a width of 2.5 mm. The diameter of the body is 24 mm.
This would lead me to believe that a 21 mm ID × 2 mm wide O-ring would work great. However, I've only ever received (and used) O-rings that are 17 mm ID × 2 mm wide. Hans Hampes tried a 21 mm ID × 1.5 mm wide O-ring and found it to be way too sloppy (but found a 18 mm ID × 2 mm wide O-ring worked very well for him). This seems to be one location where sizing the O-ring to the measurements just doesn't quite work out. I will continue to use the 17 mm ID × 2 mm wide O-rings that I've had good luck with in the past.
This O-ring is not listed in any spare parts catalog that I've seen. Viton O-rings are best, but Buna-N will also work and are a lot cheaper. Make sure to purchase an O-ring that is sufficiently soft...something with a Shore Durometer (A Scale) = 70 is great.