The only standard O-ring that comes close to your measurements (1.78mm x 9.45mm) is a ISO3601-A0095 at 1.80mm x 9.50mm. The same O-ring can be found under the DIN3771 (9.5 x 18) or JISB2401 (1.80 x 9.50 - A & G) standard.
BS1806 O-rings are likely not it. The only ones
"close" to your measurements are:
- BS1806-611 = 1.78mm x 8.74mm
- BS1806-012 = 1.78mm x 9.25mm
- BS1806-905 = 1.83mm x 10.52mm
Neither of these seem a good fit. I'd suggest you do the measurements again, possibly with the below method. It generally yields great results:
1. Take a piece of plain paper and roll it into a cylinder. The cylinder should have a diameter very close to what you expect your inner diameter of the O-ring to be. Make sure that the ends of the paper cylinder slightly overlap.
2. Take this paper cylinder and insert it into your O-ring, then unroll your paper cylinder until it sits nice and tight against your O-ring. Now make a small mark across where your paper cylinder overlaps with itself.
3. All that is left to do is unroll your cylinder, measure the distance with a quality engineer’s ruler or calliper, and divide this length by π. This results in a great approximation for the inner diameter of the O-ring you have at hand.
In the above example, we got to divide 78.94mm by π, which gives us 25.13mm. This is remarkably close to what this O-ring should be. It is a BS1806-022, which measures 25.12 mm across the inner diameter. The larger and softer the O-ring, the worse the results with this method usually become.
I find measuring O-ring diameters directly with a calliper all but impossible, except for really small ones. There is always some inherent stretch one introduces when fiddling around with the calliper.
