Getting the dimensions right is only half the job. An o-ring that is perfectly sized but made of the wrong elastomer will harden, swell or crack — sometimes within weeks. Here is how the four common materials actually behave.
Buna-N (nitrile, NBR)
The default, and correct for most jobs. Excellent with water, petroleum oils, hydraulic fluid and general-purpose sealing. Good abrasion resistance, low cost.
Temperature: roughly −30°F to 250°F. Avoid: ozone, strong oxidisers, prolonged direct sunlight, and sustained high concentrations of chlorine.
If you're sealing a pool pump housing, a hydraulic fitting or a domestic water fitting, Buna-N is almost certainly what you want.
FKM (fluorocarbon)
The chemical and heat specialist. Where Buna-N gives up, FKM keeps going: strong oxidisers, aggressive chemicals, higher temperatures.
Temperature: roughly −15°F to 400°F. Avoid: ketones, low-temperature service, hot water and steam.
In pool and spa work, FKM earns its cost in one place above all: chlorinators and chemical feeders. A Buna-N ring in a chlorinator lid sits in concentrated chlorine and hardens quickly. That's why our chlorinator lid o-rings are offered in FKM.
EPDM
The water and weather material. Outstanding with hot water, steam, brake fluid, ozone and UV.
Temperature: roughly −40°F to 300°F. Avoid: petroleum oils and greases entirely — EPDM swells badly in contact with them.
The classic mistake is lubricating an EPDM seal with a petroleum grease. Use silicone.
Silicone
The temperature-range material. Very wide operating range and excellent for static seals, food contact and medical use.
Temperature: roughly −80°F to 450°F. Avoid: dynamic applications — silicone has poor tear strength and abrasion resistance, so it is a poor choice anywhere the ring moves or rubs.
Durometer: the number after the material
Durometer measures hardness on the Shore A scale. You will see 70A and 90A most often.
- 70A — the general-purpose default. More compliant, seals well at low pressure and against imperfect surfaces.
- 90A — harder, for high pressure. Resists extrusion into the clearance gap. This is why every hydraulic kit we sell is 90 durometer.
Using 70A where 90A is specified is a common cause of "the new o-ring blew out immediately." Under pressure the softer material squeezes into the gap between the mating parts and shears.
Our standard size ranges
We stock standard catalogue sizes across AS568 (the US standard), metric, and the ISO 3601, JIS B2401, BS 4518 and SMS 1588 ranges, in Buna-N 70A, Buna-N 90A, FKM, EPDM and silicone. If you know your inside diameter and cross-section, you can order by size rather than by equipment model.
For hydraulic work, our fitting-specific kits take the guesswork out entirely — see the guides to BOSS / ORB, FACE / ORFS and flange fittings.
Before you reassemble
Every o-ring in these kits seats better and lasts longer with a food-grade silicone lubricant. Dry o-rings pinch, twist and take a set, which is how a brand-new seal starts weeping within a season.
Aqua Flex Food Grade Silicone Lubricant is what ships in our kits. Apply it to o-rings only — never to the polished faces of a mechanical shaft seal. Lubricant on the carbon or ceramic makes the seal slip, overheat and leak.
Beaver Island Parts Company is not affiliated with, endorsed by, or sponsored by any equipment manufacturer named on this page. All brand names, model numbers and part numbers are the property of their respective owners and are used solely to identify the equipment our replacement parts fit.