O-ring for sealing the 100 mm speedometer gauge to the dash (MG# 90706765)
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Buy this O-ring
The size of the O-ring (MG# 90706765) used to seal the 100 mm speedometer gauge to the single instrument dash dash is approximately 100 mm ID × 2 mm wide. Viton O-rings are best, but Buna-N work just fine 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.
McMaster-Carr part number 9262K716 will work well.