Using calipers, measure the cross-section (CS), or thickness of the rubber ring (touching it as lightly as you can, without squeezing). [3] X Research source This is the second number found in O-ring listings. Or use a ruler to measure the outside diameter (OD), or distance from one outer edge to another across the center. Since this is the same as the inner diameter plus two cross-sections of the ring, you can calculate the cross section as (OD - ID)/2. [4] X Research source
The labels usually use a standard size system. If you need to convert this to actual measurements, you can look up an O-ring size chart online. [6] X Expert Source Audra FordinCertified Automotive Technician Expert Interview. 13 January 2022. A chart for the global ISO 3601 standard should work for almost any O-ring worldwide. [7] X Research source
If the code doesn’t seem to match this system, try visiting the manufacturer website. You can often find a sizing chart specifically for their products. (In many cases, this is just the standard numbering system with some extra numbers or letters tacked on. )
Almost all standards worldwide are compatible with each other, so two -240 O-rings should be the same even if they are made in different countries. [11] X Research source
Some manufacturers add an extra number in front of the dash, for instance “3-901. " This is just a way to categorize their products, and doesn’t affect sizing. [12] X Research source If the code is a letter followed by a four-digit number (like D0425), this is a metric O-ring with no perfect match in the dash number system. You can find its exact size online with an ISO 3601 size chart. [13] X Research source If the code doesn’t match any of these, it’s probably a nonstandard size you can only buy from that one manufacturer. [14] X Research source
Try to measure the length along the inner edge of the ring for a more accurate result.
Now you have enough info to buy a replacement O-ring. The sizes are usually listed with the inner diameter first, then the cross-section. So if you measured an inner diameter of 7. 6mm and a thickness of 1mm, look for a replacement labeled something pretty close to “7. 6 x 1. "
For instance, if the groove is 5 mm, the O-ring should be at least 5 mm * 0. 95 = 4. 75 mm, and no more than 5 mm * 0. 99 = 4. 95 mm.
If you’re working with higher pressure or friction than a typical DIY project and need more precision, a good rule of thumb formula for the cross-section is 1. 4 * ((BD - GD)/2), or 1. 3 * ((BD - GD)/2) for O-rings that touch moving parts. GD is the groove diameter, and BD is the bore diameter, or diameter of the outer surface the groove is cut into. [20] X Research source
You can use silicone grease on most common O-rings, except ones made from silicone compounds. For those, try a mineral-oil-based lubricant.
