O-ring accessories

An O-ring change most often goes wrong in four places: measuring the old ring, cleaning the groove and the mating face, lubricating the ring before it goes in, and the fitting itself, where the rubber is easy to cut. Measuring is explained in the guide below; the accessories on this page cover lubricating and fitting — plus an assortment box for the repairs nobody plans for. Material and size selection belong to the catalogue and the technical tools; this page is about how you measure the ring and what you fit it with.

Ordering a replacement takes two dimensions — inner diameter (ID) and cross-section (CS). A digital caliper reads them to roughly ±0.02 mm and an analogue one to about ±0.05 mm; a ruler and a magnifier get you to ±0.3 mm, which at cross-sections of 1.78 mm or 2.62 mm is enough to order the wrong ring. Put the ring on a hard flat surface, close the jaws gently (rubber compresses easily) and repeat the measurement after turning the ring 90° — a difference of more than 0.1 mm means the ring is deformed.

Match the measured cross-section to the nearest value one of the standards actually tabulates: AS568 uses 1.78 / 2.62 / 3.53 / 5.33 / 6.99 mm, the ISO 3601-1 metric series A–E use 1.80 / 2.65 / 3.55 / 5.30 / 7.00 mm, and JIS B 2401 has its own range from 1.5 to 8.4 mm. If your reading is close to none of them, that is not a measuring error — cross-sections outside the standards are produced as a matter of course; enter such a size straight into the search. Outer diameter follows as ID + 2 × CS. For damaged or stretched rings that cannot be measured directly, our guide covers five measuring methods, including a wax imprint.

Old seal residue, dried oil and dirt in the groove are a common reason a leak comes back after a replacement. A degreaser takes the grease off the groove and the mating face immediately before assembly. Scrape with plastic or wood — a metal tool leaves a score line in the groove, and the medium will escape along it no matter which new ring you fit.

A thin film of assembly grease protects the ring as it slides over edges, threads and circlip grooves, and reduces the risk of the rubber twisting or tearing. On a dynamic seal it also lowers friction on the first start-up. Apply a thin film only — excess grease is trapped in the groove and needlessly raises groove fill, and the recommended fill band is 60 – 85 %.

Silicone grease is not universal, though. In our chemical resistance table it is rated poor for silicone (VMQ) rings — a silicone elastomer swells heavily in a silicone medium, a same-family incompatibility. For FKM the rating is marginal (some silicone greases cause a volume change), while NBR, HNBR, EPDM, CR and polyurethane are fine. Check the ring compound before you lubricate.

Most rings are damaged while the old one comes out and the new one goes in. A screwdriver has a sharp edge — it scratches the groove and the shaft, and often nicks the new ring. Assembly hooks and picks have rounded tips that lever the ring out of the groove without touching the sealing face. Cover sharp edges, threads and circlip grooves along the fitting path with thin foil or an assembly sleeve. A bearing puller belongs to the same repair when the seal is only reachable after the bearing, pulley or hub comes off.

An assortment box of common sizes earns its place as a workshop reserve for repairs you cannot plan. It does not replace the specific size-and-compound pair a machine calls for — a box covers sizes, not resistance to the medium in the circuit. If you know which sizes you replace repeatedly, building your own kit from the catalogue is usually cheaper and more dependable.