Washers do two related but distinct jobs: sealing (blocking fluid or air) and spacing (setting a gap or distributing load). Knowing which you need changes the material, thickness and hardness you should buy.
Sealing washers
A sealing washer compresses to fill the gap and block leaks. You want a softer, often thinner rubber that conforms to the surface — in a material matched to the fluid (EPDM for water, nitrile for oil). Compression is the point.
Spacing washers
A spacing or shim washer sets a precise gap or spreads load. Here you want a firmer, dimensionally stable rubber that resists compressing too much — thickness accuracy matters more than conformability.
Quick guide
| Need | Pick |
|---|---|
| Block a leak | Softer, fluid-matched compound |
| Set a gap / shim | Firmer, dimensionally stable rubber |
| Distribute load | Firmer, larger OD |
Dial in thickness and hardness with the size guide and durometer guide, then shop by size.