Direct answer: when you do not know the O-ring size, start with the old seal, the groove, or the original part information. Measure inner diameter (ID) and cross section (CS) without stretching the rubber. If the old seal is swollen, flattened, cracked, or stretched, treat its measurements as clues rather than proof.
Start with the old O-ring
Place the seal on a clean, flat surface and use a digital caliper where possible. Measure:
- ID: the opening inside the ring
- CS: the thickness of the round rubber body
- OD: a check value calculated as OD = ID + 2 × CS
Follow the O-ring measurement guide for the full process.
Check whether the old seal is still reliable
Heat, pressure, oil, fuel, water, steam, chemicals, and age can change the shape of an O-ring. Measure in several places and look for swelling, flattening, hardening, cracking, cuts, or permanent stretch.
When the old seal is badly deformed, compare it with the groove, original drawing, equipment manual, part number, or an unused matching seal.
Inspect the groove or sealing location
Useful details include groove diameter, width, depth, seal location, and whether the parts remain stationary or move against the seal. Do not force a size recommendation from incomplete groove measurements; use them together with the old part and equipment information.
Look for a standard reference
An AS568 dash number, metric size, drawing, or original part number can be more reliable than a worn sample. If you are deciding between metric and AS568 sizing, use the Metric vs AS568 guide.
Size is not the only decision
After identifying ID and CS, confirm material, hardness, fluid, temperature, pressure, and movement. Use the Material Guide and Chemical Compatibility Guide as screening tools.
Choose the buying path
- Size, material, and hardness confirmed: search the exact size or browse small packs.
- Old seal damaged or size uncertain: use Bulk Quote and attach measurements and photos.
- Drawing, sample, special material, unusual size, regulated requirement, or larger quantity: submit all available application details for review.
What to send for review
- ID, CS, and OD if measured
- Metric or inch dimensions
- AS568 or original part number if known
- Material and hardness if known
- Fluid or chemical
- Temperature and pressure
- Static or dynamic use
- Quantity
- Photo, drawing, groove measurements, or sample details
FAQ
Can I buy an O-ring if I only know the outside diameter?
OD helps, but it is usually not enough by itself. Confirm ID and CS whenever possible.
What if the old O-ring is stretched?
Measure it in more than one direction and inspect the groove or original part information. Do not assume the stretched measurement is the original size.
Can I search by AS568 number?
Yes. An identified dash number is a useful size reference, but material and hardness still need separate confirmation.
When should I request review instead of ordering directly?
Request review whenever size, material, fluid compatibility, documentation, drawing, sample, or operating conditions are uncertain.
