An M8 bolt is 8.00 mm (0.315") in diameter. The nearest stocked ID is 5/16" (7.94 mm) — just 0.06 mm under, so it stretches on for a near-perfect sealing fit. For an easy slide-on clearance fit, step up to 3/8" (9.53 mm). All sizes ship in seven rubber materials.
Best in-stock matches for M8
| Washer size | Fit |
|---|---|
| 3/4" OD × 5/16" ID × 1/16" | Snug sealing fit |
| 7/8" OD × 5/16" ID × 1/16" | Snug, wider face |
| 1" OD × 5/16" ID × 1/8" | Fender-style, more compression |
| 1-1/4" OD × 5/16" ID × 1/8" | Maximum coverage |
Browse all 5/16" ID and 3/8" ID rubber washers.
Sizing note
The 5/16" ID is effectively an exact M8 match — the 0.06 mm difference is smaller than typical rubber cutting tolerance. Choose 3/8" ID only when the washer must float freely on the bolt.
More metric bolts: M4 · M5 · M6 · M10 · M12 · M16 · M20 — or see the metric conversion guide, Size Finder and size chart.
FAQ
What size rubber washer fits an M8 bolt?
A 5/16" (7.94 mm) ID rubber washer is a near-exact match for an 8 mm M8 bolt; a 3/8" ID gives a loose clearance fit.
Is 5/16" the same as 8 mm?
Almost — 5/16" is 7.94 mm, only 0.06 mm smaller than 8 mm. On rubber, that difference is negligible.