Viton O-Ring Durometer and Dash Size Buying Guide

Quick answer

Start with the AS568 dash size, because it sets the inner diameter and cross-section that must match your groove. Then confirm the material and durometer: Viton FKM at 75 Shore A handles -15°F to +400°F and resists many fuels, oils, and chemicals that would destroy nitrile. If your groove is standard AS568 and your fluid is compatible, a 75 durometer Viton FKM o-ring is the default choice; step away only when you need a softer durometer for low-pressure sealing or a different elastomer for a specific chemical.

How AS568 dash size controls fit

AS568 dash numbers are not arbitrary — they encode a specific inner diameter and cross-section. A dash 207 o-ring, for example, has a 0.546" inner diameter and a 0.139" cross-section, while a dash 273 has a 9.234" inner diameter with the same 0.139" cross-section. The cross-section stays constant across a dash series, but the inner diameter grows with the dash number. That means you cannot substitute a dash 207 for a dash 273 just because the cross-section matches; the o-ring will not sit in the groove correctly.

To find the right dash size, measure the groove inner diameter and the groove width, then compare those to the AS568 chart. If the groove is not standard, you need a custom or metric o-ring, not an AS568 part. Buying by dash size alone without checking the groove dimensions is the most common source of leaks.

What 75 durometer means in practice

Durometer measures hardness on the Shore A scale. A 75 durometer o-ring is medium-hard — hard enough to resist extrusion into gaps under pressure, but soft enough to deform and seal against imperfect surfaces. Lower durometer (50–60) seals better at low pressure and on rough surfaces but extrudes more easily. Higher durometer (90) resists extrusion but needs more squeeze and can be harder to install in tight grooves.

For most industrial and automotive applications, 75 durometer is the balanced choice. It works with standard groove designs and typical pressures. If your system runs at very low pressure or the sealing surface is pitted, consider a softer durometer. If you are sealing high pressure with large clearances, a 90 durometer or a back-up ring is the better path.

When Viton FKM is the right material

Viton FKM is a fluoroelastomer with a service range of -15°F to +400°F (-26°C to +204°C). It resists many oils, fuels, hydraulic fluids, and aromatic hydrocarbons that cause nitrile or EPDM to swell or crack. That makes it a strong default for engine, fuel system, and chemical exposure applications.

Viton is not universal. It performs poorly with ketones, esters, and some amines, and it is not the best choice for hot water or steam — EPDM usually wins there. If you are unsure about chemical compatibility, check the fluid against a Viton compatibility chart before buying. The cost premium over nitrile is real, so reserve Viton for applications that actually need its temperature and chemical resistance.

Sizing and installation checks before you buy

Confirm three dimensions: inner diameter, cross-section, and groove depth. The o-ring should sit in the groove with a slight stretch (about 1–5% for inner diameter) and enough squeeze to seal. If the o-ring is too thick for the groove, the assembly will not close; if it is too thin, it will not seal. Measure with calipers, not a ruler.

Also check the surface finish of the groove and the mating parts. A rough surface can tear a soft o-ring, and a sharp edge can nick it during installation. Lubricate with a compatible grease before assembly to reduce installation damage. If you are replacing an o-ring, match the dash number and material of the original whenever possible.

Decision rules for buying Viton o-rings

• Match dash size to groove dimensions first: Measure groove inner diameter and width, then select the AS568 dash number that matches. Do not choose by cross-section alone. If the groove is not standard AS568, look for a metric or custom o-ring instead.
• Choose 75 durometer unless you have a reason not to: 75 Shore A works for most static and dynamic seals at moderate pressure. Go softer for low pressure or rough surfaces; go harder for high pressure with large clearances. Stick with 75 durometer for general-purpose sealing; document any deviation.
• Confirm Viton FKM compatibility with your fluid: Viton handles oils, fuels, and many chemicals from -15°F to +400°F, but fails with ketones, esters, and hot water. Check a compatibility chart before ordering. If the fluid is incompatible, choose EPDM, nitrile, or PTFE instead.
• Inspect the groove and installation path: Smooth surfaces and chamfered edges prevent nicks. Lubricate with a compatible grease and avoid stretching the o-ring more than necessary. A damaged o-ring will leak regardless of size or material.

Product facts

• 0.139" CS X 9.234" ID Viton FKM O-Rings | AS568-273 | Dash 273 | 75 Durometer | Brown (8 / Pkg.): Viton FKM o-ring, AS568-273, 9.234" inner diameter by 0.139" cross-section, 75 Shore A, brown, temperature range -15°F to +400°F. A large-diameter option for housings or flanges that need a 0.139" cross-section and a 9.234" bore. The 75 durometer balances sealing and extrusion resistance in standard grooves.
• 0.139" CS X 0.546" ID Viton FKM O-Rings | AS568-207 | Dash 207 | 75 Durometer | Black (75 / Pkg.): Viton FKM o-ring, AS568-207, 0.546" inner diameter by 0.139" cross-section, 75 Shore A, black, temperature range -15°F to +400°F. A common small-diameter size for ports, fittings, and small covers. Same cross-section and durometer as the dash 273, so you can standardize on one material and hardness across different bore sizes.

Who this is for

Best for

• Standard AS568 grooves in industrial, automotive, or chemical equipment
• Applications with oils, fuels, or temperatures up to 400°F where nitrile would fail
• Maintenance teams standardizing on one material and durometer across multiple bore sizes

Not for

• Hot water or steam service — EPDM is usually the better choice
• Ketones, esters, or amines that attack FKM
• Non-standard or metric grooves that do not match AS568 dimensions