An O-ring is the cheapest component in most assemblies and one of the few whose failure can disable the whole machine. Yet many seal failures trace back not to the part itself, but to decisions made on the spec sheet: a size standard mismatched to the groove, a hardness picked from habit, a tolerance nobody questioned. This checklist walks through the five decisions that matter most, in the order a buyer usually meets them.
1. Pin down the size standard before anything else
The most common North American mistake is quoting an AS568 dash number to a supplier with metric tooling, or vice versa. AS568 and ISO 3601 overlap heavily but are not interchangeable, and DIN 3771 adds its own dimension tables for European equipment. Before requesting quotes, confirm which standard the gland was machined to. If you inherit a drawing with only a dash number, verify the actual gland dimensions rather than assuming; converting after tooling exists is where dimensional errors get expensive. Reliable public size tables exist for all three standards — a good practice is to attach the table row, not just the number, to your RFQ so both sides are looking at the same cross-section and ID.
2. Choose material by environment, not by habit
NBR (nitrile) remains the default for general-purpose hydraulic and pneumatic work: inexpensive, good abrasion behavior, fine in oil below roughly 100°C. FKM (fluorocarbon) earns its premium in hot oil, fuels, and many solvents. EPDM is the choice for water, steam, and brake fluids — but it swells badly in petroleum oils, which is why material substitution between these two causes so many field failures. Silicone handles wide temperature swings and food-contact work but tears easily in dynamic service. Write the temperature range and the media into the spec, not just the polymer name, and let the supplier confirm chemical compatibility in writing.
3. Specify hardness deliberately
Most O-rings are 70 or 90 Shore A, and the choice is a real trade-off. Softer 70-durometer seals need less closing force and forgive surface imperfections, making them the default for low-pressure and static service. Harder 90-durometer stock resists extrusion into the clearance gap at high pressure, but it is less forgiving of imperfect finish and requires better gland control. A practical rule: below about 1,500 psi with a static gland, 70 A is usually right; above that, or in dynamic service, move toward 90 A and check the extrusion gap. Compression set matters here too — a seal that has flattened permanently in a hot gland was often the correct hardness installed in a gland that exceeded its temperature rating.
4. Check tolerances, certificates, and the supplier’s own paperwork
Three questions catch most procurement gaps. First, what dimensional tolerance applies to the size you are buying — commodity sizes carry standard tolerances, but small cross-sections and large diameters drift wider, and a drawing with tight GD&T may need inspection lots, not just a certificate. Second, which certificates come with the lot: at minimum, a material cert; for regulated work, ask about batch traceability and USP/food-grade documentation before the PO, not after. Third, who compounds and cures the material — a supplier that controls compounding can hold properties lot to lot; one that resells cannot answer the first two questions confidently.
5. Avoid the five spec-sheet mistakes that cause field failures
Finally, run your spec sheet against the recurring failure modes: gland fill too high (leave 20–35% free volume for thermal growth), an extrusion gap that exceeds the pressure rating of your chosen hardness, a size standard mismatch between drawing and purchase order, material chosen by name instead of by verified media compatibility, and surface finish outside the recommended range for the seal’s duty. A printable pre-order checklist built from these five items costs nothing to add to your RFQ template and eliminates most seal-related warranty arguments before they start.
Article by O-Ring Supply Co. engineering team. Technical references:
https://oringsupplyco.com/size-chart/
https://oringsupplyco.com/standards/din3771/