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Free Workshop Tool
O-Ring Size Finder
Measure your worn O-ring with calipers, enter any dimension and find the matching standard metric size (DIN 3771 / ISO 3601, CS 1.0–6.0 mm).
Track seal and fitting specs against every asset in your fleet — free.
Enter Your Measurements
Enter any one or two dimensions — the tool finds matching standard sizes. OD = ID + (2 × CS).
Worn or stretched O-rings may measure 10–15% off nominal. Use ±0.5 or ±1.0 for badly degraded seals.
O-Ring Material Guide
Choose the right material — the wrong one will swell, harden or fail under pressure
NBR (Nitrile)
Use for: Hydraulic oil, engine oil, petroleum fuels, compressed air — general fleet & plant use
Avoid: Water/steam systems, brake fluid, ketones and esters
Temp: −40°C to +120°C
EPDM
Use for: Water, steam, detergents, glycol brake fluid, washing systems
Avoid: Petroleum oils, hydraulic fluid, fuels — will swell and fail rapidly
Temp: −40°C to +150°C
Viton (FKM)
Use for: High temp oil, phosphate ester hydraulic fluid, aggressive chemicals, biofuels
Avoid: Steam, hot water — and higher cost, so only where NBR has failed
Temp: −20°C to +200°C
Silicone (VMQ)
Use for: Food grade, extreme temperature static seals
Avoid: Dynamic (moving) seals — tears easily under abrasion
Temp: −60°C to +200°C
Frequently Asked Questions
Use digital calipers. Measure the Inner Diameter (ID) across the inside of the ring; the Cross-Section (CS) is the thickness of the rubber cord. OD = ID + 2×CS. A worn or stretched O-ring may be 10–15% off its nominal size — use a 0.3–0.5 mm tolerance when searching.
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. A 20×3 O-ring has a 20 mm inner diameter and 3 mm cord, giving an OD of 26 mm. This is standard metric notation (DIN 3771 / ISO 3601).
NBR (Nitrile) is correct for the vast majority of fleet and plant hydraulic work — compatible with mineral hydraulic oil, engine oil, gear oil and air, handling −40°C to +120°C. Use EPDM for water and washing system seals. Upgrade to Viton only where NBR has failed.
For a static face seal, a ±0.2–0.3 mm CS difference can work temporarily. In dynamic seals (cylinder rods, rotating shafts) even a small size difference risks premature failure. Always use the correct standard size where possible.
Wrong material, wrong size, age and ozone cracking, a damaged groove, and over-torquing a fitting which crushes the O-ring flat are the most common causes.
Yes — always apply a thin film of compatible lubricant. Petroleum jelly or O-ring assembly grease works for NBR in oil/hydraulic applications. Use silicone grease for EPDM and Viton O-rings. Never use petroleum products on EPDM — they will swell it.
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