Motoscrubs.com

I'm well onto finishing a rebuild of 1965-66 Sebring motor and have been stumped by now needing the gear selector O-ring in the engine case. The Ducati part number is 0400.49.270 and it is nominally 16 mm ID. I would love to order a couple from suppliers in the U.K or Italy that I have regularly used, but the minimal cost of the seal and the high cost of mail, plus the waiting time for delivery does not make this approach sensible.
I have multiple seal sets from Ariete, but not this part.
If any of you have the dimensions of the seal including thickness to reveal, I would be most appreciative. I'm fairly sure I can order a metric O-ring pack from McMaster Carr industrial supply here in the USA and get it in two days. At least I hope.

Ducati parts list says the part description is OR 121.

This looks like an Italian UNI standard.
See here, OR 121 is mentioned in the list, with dimensions:
https://www.thisoldtractor.com/o-rings_uni.html

Thank you for the link Jordan, a handy reference to have.

Good health, Bill

Spot on, Jordan, and thank you ! I have not been on the This Old Tractor site for some time. Taking dimensions of rubber rings with a standard caliper is guesswork at best. These products are described to the hundredths of a mm.

OR 121 has a 2.62 cross section and an ID of 15.88 mm, just under the selector shaft diameter of 16mm.
For those in America here is a link: https://www.mcmaster.com/metric-o-rings/id~15-880mm/