O-ring for cylinder studs (MG# 90706090)
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
The size of the O-ring (MG# 90706090) used to seal the cylinder studs is approximately 9.6 mm to 10 mm ID × 1.6 mm to 1.8 mm wide (I've seen several sizes in that range sent to me by dealers). 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. Six O-rings are used per side; 12 total needed.
Here is one place where the inch equivalent will work perfectly fine (and is probably what some dealers are using and sending out, anyway). Just use the Dash size -012 with a Shore Durometer (A Scale) = 70. These are 0.364 inch ID × 0.07 inch Thick and will work perfectly fine. McMaster-Carr part number 9452K21 (Same as McMaster-Carr part number 9452K342) works great.