Durometer is the hardness rating of rubber, measured on the Shore A scale from 0 (softest) to 100 (hardest). Most rubber washers are molded at 60–70 Shore A — firm enough to hold torque, soft enough to conform to surface imperfections and seal. Here's how the scale works and how to pick the right hardness.
The Shore A scale with everyday references
| Shore A | Feels like | Washer behavior |
|---|---|---|
| 20–30 | Rubber band | Very conformable, squirms under bolt load |
| 40–50 | Pencil eraser | Seals rough surfaces, limited torque |
| 60–70 | Tire tread / shoe heel | Standard washer hardness — balanced seal and load |
| 80–90 | Shopping cart wheel | High clamp loads, needs smooth surfaces |
| 95–100 | Hard hat / bowling ball edge | Approaching rigid — spacing more than sealing |
How durometer is measured
A durometer gauge presses a spring-loaded indenter into the rubber; the resistance reads out 0–100. Shore A covers flexible rubbers; Shore D takes over for hard plastics and ebonite. A reading of "70A" means 70 on the Shore A scale.
Why washer hardness matters
Sealing: softer rubber (50–60A) flows into scratches and casting texture, sealing imperfect surfaces at low torque. Load-bearing: harder rubber (70–80A) resists extrusion under high bolt loads and doesn't squeeze out of the joint. Vibration: softer damps more but fatigues faster under big amplitudes. The 60–70A of our standard washers is the engineering sweet spot for fastener sealing.
Typical durometer by material
As commonly molded for washers: natural rubber ~40–60A (softest, best rebound), neoprene ~60–70A, EPDM ~60–70A, nitrile ~65–75A, silicone ~50–60A (stays soft across its huge temp range), Viton ~70–75A, and vulcanized fiber effectively off the Shore A scale — it's a rigid material used for spacing and insulation.
Note durometer only rates hardness — it says nothing about chemical or temperature resistance. Pair this guide with the material comparison to choose on both axes.
FAQ
What durometer are standard rubber washers?
60–70 Shore A. It seals typical machined and stamped surfaces while tolerating normal fastener torque without extruding.
Is higher durometer better?
Not inherently. Higher hardness handles more clamp load but needs smoother surfaces to seal; lower hardness seals rough surfaces but extrudes under heavy torque. Match hardness to surface finish and load.
Does temperature change durometer?
Yes — rubbers harden as they age and when cold, and soften when hot. Silicone changes least across temperature, one reason it's used from −85°F to 400°F.