Question 1

How do I measure an O-ring accurately?

Accepted Answer

Use digital callipers. Measure the Inner Diameter (ID) across the inside of the ring; the Cross-Section (CS, also called cord or section diameter) is the thickness of the rubber cord itself. OD = ID + 2×CS. A worn or stretched O-ring may be 10–15% off its nominal size — measure it and use a 0.3–0.5 mm tolerance when searching. Lay the O-ring flat and measure across the inside gap for ID.

Question 2

What does "ID × CS" mean on an O-ring?

Accepted Answer

ID is Inner Diameter — the hole inside the ring, in mm. CS is Cross-Section — the diameter of the rubber cord that forms the ring (also called the section or cord diameter). A 20×3 O-ring has a 20 mm inner diameter and 3 mm cord, giving an outer diameter of 20 + (2×3) = 26 mm. This is the standard metric notation used in the UK and Europe (DIN 3771 / ISO 3601).

Question 3

What O-ring material should I use for hydraulic fittings on farm machinery?

Accepted Answer

NBR (Nitrile) is the correct material for the vast majority of farm hydraulic work. It is compatible with mineral hydraulic oil, engine oil, gear oil and air. It handles temperatures from −40°C to +120°C, is inexpensive and is what most comprehensive assortment kits contain. Use EPDM for water and washing system seals. Upgrade to Viton (FKM) only where NBR has failed and you suspect high temperatures or aggressive fluid compatibility issues.

Question 4

Can I fit a slightly different size O-ring if I cannot find the exact match?

Accepted Answer

For a static face seal (e.g. a union or fitting that does not move), a ±0.2–0.3 mm difference in CS can work temporarily. A slightly oversize O-ring will be compressed more tightly; a slightly undersize one may not seal under high pressure. In dynamic seals (hydraulic cylinder rods, rotating shafts) even a small size difference risks premature failure. Always use the correct standard size where possible.

Question 5

What causes O-rings to fail on farm machinery?

Accepted Answer

The most common causes are: wrong material (EPDM dissolving in hydraulic oil, NBR swelling in water); wrong size (over-compressed O-rings extrude under pressure, under-compressed ones leak immediately); age and ozone cracking (O-rings harden and crack outdoors); damaged groove (wrong depth or a scratched groove destroys sealing); and over-torquing a fitting which crushes the O-ring flat.

Question 6

What is a BS1806 O-ring number?

Accepted Answer

BS1806 is the British Standard for O-ring sizes, grouping them by cross-section: BS001–030 have CS 1.78 mm, BS031–049 have CS 2.62 mm, BS050–073 have CS 3.53 mm, and so on. The number does not directly correspond to the ID or OD in millimetres. If you have a BS number from a parts manual, look up the exact dimensions in a BS1806 cross-reference table, then use the metric ID×CS notation to source a replacement.

Question 7

Is an O-ring assortment kit worth buying?

Accepted Answer

For a farm workshop, yes. A comprehensive metric NBR assortment kit covering 200–400 rings from CS 1.5 to CS 5 mm costs £15–40 and covers the majority of hydraulic fitting, valve and union leaks you will encounter. Ordering individual rings costs more per ring and involves delay. Buy a quality NBR kit as a base stock, and add a smaller Viton kit if you work on older machinery or high-pressure chemical systems.

Question 8

Should I use O-ring grease when fitting a replacement?

Accepted Answer

Yes — always apply a thin film of compatible lubricant before fitting. Petroleum jelly (Vaseline) or O-ring assembly grease works for NBR in oil/hydraulic applications. Use silicone grease for EPDM and Viton O-rings (silicone grease is compatible with all rubber types). Never use WD-40 or engine oil on EPDM O-rings — petroleum products will swell them. Lubrication prevents the O-ring rolling in the groove and tearing during assembly.

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